Err... It got strictly
nothing to do with voice.
The media filter only applies to media (parcel media, parcel music, media on a prim), not to voice (which is not even connecting to the server via the viewer, but instead via SLVoice, itself having no relation whatsoever with the media filter code) !
This however could be some weird interaction with your DNS stack (faulty/buggy DNS cache daemon): try re-enabling the filter and disabling "Lookup IP address for filtered domains".
Maybe this could cause your DNS cache to stay stuck, somehow...
No, but you loose the filter protection...