While shooting? Strictly my own pulse. Shooting for me is about concentration rather than distraction.
While handloading or cleaning guns, I usually have on classical, Nordic pop/folk, Celtic, medieval/renaissance (Jordi Savall esp.), harp and lute (also mostly classical), liturgical chant, Tuvan throat singing, modern (Glass, Reich, Adams, etc.), pop country, geezer rock, some jazz. My music tastes pretty much run the gamut, except for really annoying stuff like Fado, rap, Muzak, opera or Mexican folk singing.
When charging cases with powder I limit vocals to singing in languages I don't understand -- Gregorian chant and Georgian polyphony are both good. No temptation to sing along.