Has anyone tried Voice dialing using a Sony T608 Bluetooth?

You shouldn't have to re-record them unless, of course, the difference in sound quality is so different that the phone's voice matching can't find the right contact. If you plan on voice dialing with the Prius more often than with the phone itself or with a headset, then re-recording them using the Prius mic would probably improve the matching.

On the phone itself, and with a BT headset, pressing either the volume up or volume down buttons should cause the phone to beep once prompting you for the name of the contact. You should then say the name, and the phone should repeat your recorded name for that contact once it's matched, and then it will automatically place the call.

I haven't yet tested whether or not the +/- volume buttons on the touchscreen actually do the same thing or not. I have tested, however, that my Bluetrek earpiece (BT connected and only $40!) does indeed accept voice commands by pressing the volume buttons on the headset, so if the Prius software was designed properly, it should do the same thing.