There are a lot of musical instruments in Heartlands already, but they all have purely a cosmetic/roleplaying function.
I'm suggesting to consider taking it to the next level. Let's imagine we have a menu option (similar to emotes), titled "Sound Calls", and instruments called "Blowhorns".
1. Simple blowhorns.

Craftable with ease (for example, few boar tusks, bone material and glue).
Usable as a musical instrument; allows to activate Scattering Howl and Friendly Tune menu items***.
- Scattering Howl: forces all wild animals in an "X" tiles area to flee for a few seconds. Cave Bats may flee double the time. Can't be activated more than once in "Z" minutes (used to clear the desired area of possibly hostile animals like lynxes, bears, boars or bats).
- Friendly Tune: all characters in the vicinity (for example, 2 grids) receive a short-timed buff "heard friendly tune from X", with a tracking widget to the point from which the tune was played, remaining for the length of the buff (possibly CONST-dependant for user, PERC-dependant for receiver). If a person playing the tune is in the kin list, the buff should say "heard a friendly tune of [nickname]', otherwise it's just "someone" (as with PClaims). Used to announce your non-hostile presense in the area.
***possible implementations of other sound signals, effect depends on user's constitution (and probably psyche).
2. Advanced blowhorns.

Those would require some expensive materials (like precious metals, pearls and such), but would allow the user (provided he has enough CONST, CHA and PSY) to use various Sound Calls, for example, "Call To Arms" boosting his party members moving speed for a few seconds but having his own speed reduced for the same time. You get the idea: the leader blows horn, boosts his company somewhat but impairs his own abilities for some time (ofcourse, with appropriate de-buffs to avoid spamming).
Also, there could be a Call for Help; a Call of Triumph and other sound communications, avaliable to use if a person have enough stats.
Besides obvious uses for mass combat, I believe a lot of sieging/raiding uses could be developed here. We could even go as far as shoo-ing all the domesticated horses in the area, dropping their riders to the ground for a short period of time (with implemention of a "collapsed lung" debuff for the user, damaging him for some HHP and holding him unconscious (therefore, preventing him to use any Sound Calls) while the debuff holds).
As for the endgame, I'd suggest a special Sound Call, "Resonating Roar", usable only inside a cavern with a check of the user's strength (should be enough to mine all tiles in the area of effect), that would re-activate all the "dropping sand" spots to indicate where a cave-in would most likely occure; have a small chance to damage any Mine Support/Stone Column in the area for 10%/5% total damage, and have a 0.5% chance to convert any tile in the effective radius (basically small, but CONST+PSY dependant) into a Restless tile, which, upon being mined, have a 10% chance to spawn a Troll or 50% chance to convert one adjacent tile into a Restless one.
Ofcourse, a use of Resonating Roar call should inflict a "Soundwave concussion" debuff on it's user, preventing him from using the Resonating Roar again for some time (yet again, CONST-dependant).
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Well, here it is. Go on and crap all over those ideas =)