while it's certainly a possibility that there is a manufacturing defect at play here which could be further investigated, i'm going to go out on a limb and say that it seems unlikely if other audio, ie: calls sound ok, and only voicemail has quality issues. especially since there seem to be at least a handful of us experiencing it. (it's like my calls are 256k encoded mp3s, but voicemails are only 64k or something)
my guess is still that it's an ATT backend operation, and it may be a decision on a regional basis, or is maybe even more random like the new version of whatever they use to encode VMs just does a crappier job.
Perhaps we should ask where the people experiencing this issue are located. I'm in central indiana.