(you know the old joke:
Question: If a bagpipe and an accordian fall from a twelve story building, which one hits first?
Answer: Who cares?)
(I'd be amused to see what the party would do--I'll bet after your first description of the cacophany, they'd be ready to slaughter all bagpipes indiscriminately. Would they have pity on one really fine instrument? Would they even stop to look, or head right on by--they're very busy people these days . . .)
(Or imagine, Nolin's instrument demanding the destruction of the poor thing, on point of principle.)
Something really creepy would be a ordinary human teenager that is an evil prodigy (I suppose prodigy means very gifted child). Make the child as a freaked out Lolita- kind of 15th lvl necromancer/enchanter who runs an evil guild or organisation that have a strong position in the trading post.
Especially the enchantress kind of teenage evil can make for a very grotesque cross between forbidden lust and fear for the people she meets, something that she uses all the time.
You could make her fake a repentance, and then backstab the PCs. Or cast a still, silent charm person on one of the PCs (Nolin would be the perfect victim) and make the other PCs be disgusted with the PCs behaviour.
Now, if you prefer a less dark mood (or more Eric's grandma ) the teenager can be a shapechanged something, but that isnt as unsettling IMO.
Oh man, that'd be funny. Have a "museum" from the point of view of the bad guys; for example, one exhibit would be a 10x10 room with a lone Orc (illusionary) guarding a chest, with a little plaque outside commemorating the valiant death of Kurg the Magnificent at the hands of an evil party of surfacedwellers (a Cleric, a Wizard, a Fighter, and a Rogue). I'm sure someone could come up with more cliches to use.
Thing is, the above obscene doodle of a bunch of stark naked kobold ladies playing wind instruments (apart from the kinky one who's wearing a kilt, and the fat lady playing the drum) has garnered a certain sentimental popularity, resulting in souvenir brassware being sold depicting the infamous fresco.