Can't transfer dubbed audio (secondary sound) using firewire.
Audio Dubbing on DV:
Here's how you do this: Play your tape on the TRV-9 and feed the signal into the TRV-900, using the Firewire connector. Set the TRV-900's audio option to record in the 12-bit mode. When this is done, you will have the video re-recorded on the TRV-900 and the audio laid down on the primary 12-bit audio channel. Then, you can run the tape back through for a 2nd pass in the TRV-900, with the recorder set to the audio-dubbing mode. The audio you put into the TRV-900 this time, will be recorded on the secondary 12-bit channel. You can select either 12-bit channel when you play the edited tape back or you can mix both stereo channels into the output.
Once you record on the 16-bit channel, you can't dub audio back onto that recording, without erasing the original channel, as the recording space for both 12-bit channels is occupied. You can't selectively re-record just audio onto the 16-bit channel either, without the video being re-recorded along with it. Both video and audio are sent over the same Firewire connector, so you can't input them from mixed sources, as you can with analog recordings.
Only the secondary 12-bit audio channel can be used in the audio-dubbing mode. You can't dub onto the 16-bit channel or onto the primary 12-bit channel. The primary 12-bit channel can be used only when you're recording video along with it.