So far my party is in a town investigating a disappearing adventuring party (Their former character). They have located the area in which the players last saw their characters and are currently trekking into a hobgoblin-occupied settlement to find information. How do I make the interactions between the hobgoblins more than just a hack-n-slash and give more meaning to society? There is a whole story of corruption and ancient cult/ritual stuff... you know... classic D&D plot, but I don't want to overload this "side quest" adventure to a point where they forget about the main plot, you know? 

As they trek in,  do not put up a battlemap.  In my experience, for w/e reason (psychology) when players see a battlemap they then stop narrative and move to attack mode.

I steal concepts from other games and systems.  In your case, I would have every player roll a d20, then you as DM use that ranking as an "initiative".   Each character in said initiative order then controls a scene and defines the interactions. Once they specify what they are trying to accomplish,  I as a DM come up with the appropriate roll -- then improv from there :)  You can use this to provide exposition where you aren't just reading, but instead the players/characters are interacting with the world.

This is NOT directed at OP, but for the gentle readers..  Some DMs have  a hard time letting go of the reigns and letting players really help create the narrative and shared world.   It's a different kind of work and challenge for the DM.  That style doesn't work for all games though, so make sure you know what type of game you are running.

Just a thought :)

If you're looking for actual suggestions -

Whatever any given DMs goblin lore is - Hobgoblins respect strength and they enjoy order.  Kiss up, kick down, Lawful Evil.  If your players speak with the voice of some other higher authority...a government they recognize, an army just one valley away, anything to plausibly suggest that attacking the characters on sight could end up leading to an even bigger  headache down the road...they'll get a respectful hearing.  That gives you some time to bring them into the side quest.  That's as much help as I can give without knowing more about the side quest story and why the players would care?

Koradgee  -that sounds interesting.  Is there a stream where I could watch a technique like that?

Apologies as I didn't really give an explicit  suggestion.

How do I make the interactions between the hobgoblins more than just a hack-n-slash and give more meaning to society?

My intent was to turn this into an RP / skill challenge/ narrative encounter. I read this as:

Goal:   How do I make the players get interested in this hobgoblin society and potentially important lore.

Solutions:  

• Let the characters interact with hobgoblins in non-combat ways.
• As Tim mentioned, use those interactions to define the society and show its rules, order and structure.
• As you seem to already have some plot hooks ready,  drop those in as part of the characters interacting with the society. To my original comment, let the characters experience and participate in the lore/story instead of it just getting read to them.

Tim, I don't watch a lot of streams (I'd rather play/run/prep dnd than watch others for long -- thoughI do watch on occasion to steal concepts).  I think the closest analogy to what I described is "ten candles", which is a very narrative game, but letting the players create some scenes tends to increase the investment in the world... and offloads work for the DM.

If they are going to the Hobgoblin settlement for information, is the information something the Hobgoblins know or is it something they have? And importantly, do the characters know this?

Without knowing exactly how you have set it up: are the characters heading there to talk, or to steal? If it is not the former, try and make it so. That way the players know to keep their weapons sheathed (at first).

I would suggest some kind of feat of strength or prowess to impress the Hobgoblins enough to get an audience with their Warlord/Chief. If your players like a bit of hack n slash, this could simply be defeating a large beast in an arena the Hobgoblins have set up in their settlement.

Or you could make it a little more fun and lighthearted, such as the biggest character (barbarian perhaps) having to defeat the Hobgoblin champion in an arm wrestle as a Strength skill check.

I also agree, avoid getting a battle map out on your table (physical or virtual) because that does put players into "tactical combat mode".

EDIT: if you wanted to link the "side quest" to your main plot, you could surprise the players and their characters by revealing that the warlord/chief of the hobgoblins is a member of you "main quest" cult.

Or, to give "more meaning to society", you could have the Hobgoblins opposed to the cult in some way, so there is an opportunity for the characters to gain some unexpected allies. Make this slightly oblique so that it is only through a correct approach to this as a social encounter not a combat encounter that the characters gain this advantage. For example, rather than the hobgoblins being already in conflict with the cult, perhaps their goals mean they are likely to come into conflict with the cult in the future.

First of all I never TPK a party simply because they did there own thing, also, if the players love the area and want to make it the main quest, let them. I have found the best campaigns I ever ran came from the players going off on a random tangent and completely ignoring the well crafted long term campaign I thought would grab them. 

In terms of your world I would say an out of game conversation, explain that in your world the characters would know that not all green skins are evil or need killing. Have some NPCs in the story talk about the peaceful hobgoblin villiage. Run the hobgoblins as you would any other NPC, for me that would mean roleplaying them and making them likeable. Personally my players like to roleplay social encounters with minimal dice rolling (maybe the odd insight roll or deception etc) but I always set the DC based on what the players actually physically said or did. But a villiage like your describing, I know my players would want to end up spending sessions there finding stuff out, getting to know the cultures and personalities in this situatuon they have never experienced before. If they do then try to resist the urge to railroad them out if it back to the story. 

One Motto of Hobgoblins is "Suffer nor give insult" so you could use this to play a extremly tense situation, in which the characters have to talk their way trough, and negotiate for the information they need. To prove worthy they maybe have to win a mix of highland games, lumberjack competition and american gladiators, as an arena-/deathmatch would be a bit boring. ;) Meanwhile they could also rely on spying to get the information, in case they will lose, or get the opportunity to save one of the hobgoblin contestants who took a bit too much of a risk and get the needed intel from him - if they lose and get kicked out of camp.

One suggestion would be to highlight the non-combatants of the village. As the party approaches they come across two hobgoblin children playing. Their mother, upon seeing the heavily armed party, scoops them up and rushes them into a nearby dwelling. Something like that to highlight the innocent families should make the party less intent on combat. Coming out to meet them is a hunched old hobgoblin woman or otherwise non-threatening character. Shepherds, merchants, a Bard or an artist being their first contact should help set the tone.