I already had Bill Shultz's CV-580 soundset installed, so that's what I'm using. Tweaking the volume(s) in Options/Settings/Sounds gives me what I want.
They aren't correct for the PT6 engines, but, I really like their lower "note" a lot better than the available defaults.
"FS2002/FS2004 Allison T-56 Engine Sounds", 580sound.zip @flightsim,
and the update; "T-56 Allison Turboprop Sound Update for L-188 Electra, CV-580 and P-3 Aircraft", t56_sound_update_cielo.zip @ Avsim.
There are other soundsets proffered in the BT-67 thread at Avsim:
http://forum.avsim.net/topic/378810-basler-bt-67-turbo-conversion-dc-3-released/page__st__25 ...Don