I never like blindly suggesting software upgrades, EXCEPT in the case of voice problems through the PIX. There have been numerous voice/nat/pat problems fixed up in later versions of PIX code, versions AFTER v6.2(2) I mean.
Open a TAC case and ask for the latest interim code release of the PIX (currently 6.2(2.138)), explain your problem and they'll send it to you. This'll probably fix up all your voice problems.