How about an enchantment that you can apply to the instruments the "bardic enchantment" or something like that with variations depending the instrument or the enchantment maybe?, while any entity is playing an instrument with this enchantment every other entity of the same type (including players) gains some buffs (like potion effects) speed, strenght, health regeneration, fire resistance or whatever you like just one at a time, to balance this you could maybe make instruments with the enchantment applied have durability that is consumed depending on the ammount of entities that its buffing

if this ever gets added, it would be cool if this was innately true with mobs, as it'd add a slight bit of depth to combat (focus down their healer/support and then focus the crowd)