I'm also fairly bad at doing voices while GMing, though if it's a single NPC who's chatting with the PCs I try to throw in an unusual vocal pitch or accent, or something along those lines. I'm playing at the moment with an on-line group which releases its recordings, and I'm trying out accents for my PC there.
Singing is a bit odd to me, with "the audience liked your song" as the defined success state rather than "the song was well sung", and if we'd had complementary skills when 4e was being written (to allow technical quality to be separated from audience-pleasing) this might have worked better. Many opera singers and fans will tell you that not knowing the language (as performer or audience) doesn't necessarily impair the quality of the performance, and
completely meaningless words can still be sung well; is that really at a -4?