Ospos Dex save (DC 15?): 18

As the worm reacts to the thrown gem, Ospos will take his full movement (and action to dash) to re position himself into a better position to throw rocks into the hoard room beyond the doors. 

Ospos and Nean: Between the two of you, you make enough noise around the doors to send the worm arcing out of its latest burrow, towards the one it emerged from . As it's now using already cleared tunnels, there's no burst of loosened stones. The worm dives into the ground before exploding out into the next room, causing a rain of coins and treasure as it emerges.

Everytime the worm moves away chasing the sound of thrown rocks or gems, Ospos will take the opportunity to also move towards and into the hoard room.  Once there, he will look for other ways out.

Perception: 9

"Okay, diverting it with sound appears to be working. We better do some quick thinking now guys, im almost out of steam and most of you don't really seem to be in much better shape." Nean whispers to the group ever so quietly, now that the worm is busy for a moment.

"Ospos, what are you doing? Anyway, we need to find a way to get out of here. Did anyone see another path out of that big cave we just came from? I really dont think we can take on that purple danger noodle alone." The rapid de- and increase of health might have taken a toll on his spirits for a moment.

Ospos: As magnificient as the piles of treasure are, they cover much of the walls, obscuring possible exits. 

Ospos answers Nean, "There was another way in that other cavern, but the way back has collapsed.  So I'm looking for another way out this way, that's what I'm doing."

Shit shit shit, a dead end!  Hope that tunnel hasn't fully collapsed.

Ospos will look for the mirror without actually shifting through piles of gold (make no noise, other than throwing gems or rocks far away).

Perception to find the mirror: 21

In quite the distress now, Nean begins to quietly mutter the groups option to himself. "Okay, both ways up and down have collapsed. No passing through, only this room and what's probably the lair of the worm. One part of the door still dangling around. A huge worm that can be lured with sound. Sound...."  For a brief moment, his voice fades away, his thoughts running wild. "If only we could produce some sort of consistent noise.......Okay, listen up. I know that all of these sound desperate, but i think these are our best option:

First, we can try casting out the worm. Most of the creatures living in caves dislike light in general, i hope that fire will have the same effect. It might be one of its weaknesses, or at least something it considers a thread, afterall the worm is living underground in an only restrikted space. I have 9 torches left, together we might be able to either light up a big sort of bonfire to scare it away, or scatter the torches on ground and entrances to block its path. With this option, however, we will run out of air to breath at some point, so we would be required to find another way out.

Second, and i know this sounds crazy but i still prefer it over option number three. If anyone of you is capable of playing an instrument, tell me now. Ospos has proven the worm to be sensitive to sound. It probably uses it to orientate itself. That said, i highly doubt this creature has ever heard anything close to real music. The main noises around here must have been the kobolds trying to empty the mine, so we do not yet know whether playing something that is not actually painful to hear might change the beasts behaviour. It is only a small chance, but i have heard of people hypnotizing snakes with the sound of a flute. If everything fails, try and sing! It is somewhat unfortunate that our Bard has gone to pay the monster a visit, but i will tell him the same. Although i do realize he might try and kill me for it.

The third and last option i can think of would be to fight the worm, and i would highly discourage anyone from trying. If we need to do so, we need can try and lure it to a spot that is most advantageous for us and the least for the worm. We could try and make the other metal door fall on top of it, but constructions are not my specialty. And remember that fire might be more ressourceful than other means of damaging it. Anyway, i don't believe we stand much of a chance here. If anyone else has an idea of how to handle this, let's here it, and rather sooner than later. You can see that im just grasping at straws as well." With that, Nean finally shuts his mouth and eyes up the rest of the group, with what he hopes to be a rather an engouraging than ludicrous smile.

Ospos: Your eyes dart to the ornate golden frame instantly. It's half buried in a nearby pile, but you can see intricately carved filigree depicting curling vines and tiny flowers growing around the mirror. (you would have to get closer to it to investigate any further)

Ospos will continue to throw items as far as he can to distract the worm.  When the worm moves or shifts towards the noises, Ospos will move towards the mirror.

((Let me know if I need to roll something to get to the mirror.  If I get there...))

Ospos investigates the mirror: 18

Ospos: The ground shakes as the worm moves underneath it, distracted by the sounds for the moment. You creep up to the mirror, and clear away some of the  coins surrounding it. It's beautiful, and you now notice there are also small winged humanoids carved in gold affixed around the mirror. At the top, there is an inscription written in what you recognize as Sylvan, though you can't read it. You also notice that despite its ancient age, the mirror is in perfect condition, missing the tarnish that most of the other treasures in here have had.

Ospos casts Comprehend Languages as a ritual and reads the inscription on the mirror.

Ospos: The inscription reads: "For my dearest Light of Dawn, Gaze at your beauty and see what I see; a window to paradise. Love, Caeldrinn"

Clermar whispers, "It's risky, but could we try to get that thing to burrow us out? lead it over to the entrance here and see if it will clear the debris. Then hopefully the way we came isn't too collapsed to get out." 

Ospos continues to use distractions to cover his actions.  First he frees the mirror from the hoard, then he prepares it to be quickly picked up and carried.  He'll cause a distraction, pick up the mirror and move then set it down.  He'll repeat this process until he can get to the rest of the party where someone else can carry the mirror.

((Let me know if I have to make any rolls))

Ospos: (Give me a stealth check, with advantage because of the distractions)

Ospos Stealth check: 16

Ospos: The shaking increases as you begin moving the mirror towards your companions. Turning back, you can see a mound of displaced earth trailing behind you as the worm attempts to track your movements, but it swerves and turns erratically as it is distracted by the gems being flung across the room, causing small cascades of coins here and there. At last you make it into the workroom, dragging the priceless mirror. The worm can't be seen, but tremors occasionally still shake the cavern.

Ospos puts the mirror down and says, "This is too awkward for me to carry far."

Then Ospos will inspect the tunnel leading back to the chamber with the dead dragon.  Is it passable?

Inspect tunnel: 4

Ospos: You gather that you'd need to move some of the stones to make it through, and doing so would surely cause the worm to attack, as would the whole party moving through it.

Making sure Dril is ok, Bram turns to answers Nean, trying to keep his voice low but audible,

"That thing is eating through the solid stone like it was mash potato. You really think a fire would stop it? I mean, we could try, but like you said, we would run out of air fast, with a bonfire going."

"As for making music, discordant or melodious, unless I knew for sure it would work, I'm not doing it. If that thing couldn't care less about my music, and just decides to gobble me up; no I don't think I'm going to risk that."

"I agree whole heartedly with you that I don't think we can fight that thing. I mean, it is literally eating rock like it was candy."

Hearing Clermar's suggestion, Bram remarks,

"That might work, it would probably be slow going, but we might be able to get the worm to work for us. It looks like Ospos is having some success keeping the worm in the other room."

As Ospos emerges from the other room with an exquisite mirror in hand Brams eye brows go up,

"Wow, way to go Ospos! While we sit around talking, you have managed to both keep the worm from killing us all, and have found and recovered the mirror."

"You are a truly exceptional individual!"