Nicholas, what Sony told you holds water… there’s no way firewire can separate audio bits from video bits in the data stream. There’s gotta be something on your end. Something as simple as not having audio active when in the record tool? System specs? It could be anything. I also know a few that have the same deck and they don’t have these issues.
Terence, the deck that replaced the M15U, the M15AU has 24p support also, as well as being able to playback Canon’s 24F modes (or so I’m told)