If you are talking about syncing the sound with the mng and wav outputted from mame, I believe it does not work (yet?).
The problem I think is that the MNG recording must be started manually - you can't turn it on from the command line. If you could, the mng's would be perfectly synced with the sound.
Personally I like and use sound in the avis, so using the MNG process does not appeal yet.