Refreshing this thread, 'cause it (the singing inhibition thing) is something I've often wondered about.
I have an untrained voice, mostly (Took classes on public speaking and oral interpretation in college, and was given vocal exercises to do there, but I've never been to a voice coach). But I sing almost all the time. It's like my brain is playing background music to my life, and it "leaks" ;-).
People have occasionally commented, but it's usually along the lines of: "You're always so *cheerful*!" (If they're thinking "You're crazy!", they're too polite to say.)
I'm active in the local Neo-Pagan community, and there is lots of chanting, and drumming, and even wild, strong whooping and cheering during ritual, so it's not like folks there are afraid of making noise, exactly, but there is very little actual *singing* -- hardly any melody.
I think there's a fear of hitting a wrong note. Maybe next time I go to a gathering, I'll bring some kazzoos -- the overtones and buzzes are so rich in those things that not hitting a pitch exactly doesn't seem so harsh.
If I can get folks humming, maybe I can get them singing next....