I think the spell should have a trigger for the reaction, something like "when a loud noise happens within 30 ft of you."

This is such a great idea for a spell! It especially fits very well to how I like playing my bards. The whole feeling of honing the blades of others with your tunes and notes makes it perfect. Some advice I would give is to add a trigger for the reaction. Some spells you can see this being done in are Shield or Absorb Elements. DnDBeyond has a very simple box for it when editing the casting time of a spell. Having a clear definition of when you may trigger the reaction to cast this spell is very important in order to stay in the flow of combat. I would assume for this spell the description would be something along the lines of: "1 Reaction, which you take when you see a creature within range make an attack with a held weapon.". The wording in this is important. In my example I assumed that the spell can only be cast on physical weapons such as swords or bows(/arrows), but perhaps you want the spell to also work on natural weapons for your Druid allies in which case this should be made clear in the trigger description.

If you figure this description out it will make the spell so much easier to use as it will reduce the amount of questions players may have for their DMs which in turn slows down combat. With it all cleared and laid out in front of players I feel this spell would perfectly fit into combat as a quick bonus that won't even be a bump in the flow of the game. I'm looking forward to how you envision the spell being triggered and using it in my own games, good luck!