For completeness: Pete Seeger suggests that one can muffle a banjo by stuffing a towel up the back. (That won't work with my horror, as there's a girt big wooden reflector/resonator in the way.) It's related to using a sock in the horn of a wind-up gramophone as a volume control.*

Muffling bagpipes is a whole new challenge, which I leave as an exercise.

* Which is where "put a sock in it" comes from. Strange but true.