Capturing via Firewire is the best option. That will transfer the digital video and audio from the tape to the computer. If you have to buy a Firewire card for the PC make sure to get one with a Texas Instruments chip.
Regarding your missing audio, did you plug in the red/white audio cables into the capture device as well as the video? Or you may have to select the audio input within the capture software (it may default to microphone or line-in rather than the capture device).