well you'd need some sort of audio capture device... in general probably using a device made for hardware... some examples are el gato's eye tv, or formac's studio, etc. what type of system do you have? if you have a powermac, you could theoretically get a new eyetv pci card that allows you to capture video/audio... that is under $100 i think, the other hardware devices are more like $200+
i'm guessing that if you just have a tape or two you want to do this with, that type of investment isn't worth it. try to find someone who has the capabilities and could do it for you. if not, you may have to buy the hardware or pay a company to do it (which might be cheaper if you just have one tape). heck, you could probably find someone here on macrumors who would do it for a couple. (myself included) hah
let me know if you need more specifics, and let us know your system and other details so we can give more specific ideas