speed" (sample or frame rate) of your audio recording device is unimportant as long as it is constant (crystal or electronically controlled). If you're recording a shot that is 5 seconds long in real time, it will be five seconds long on video and five seconds long on your DAT, so you can sync it.
Where you run into problems is if you change the relationships between pix and sound when digitizing, compressing, transferring or editing.