This may be a daft question revealing a complete failure to understand the technology, but:
If I get a new phone which has an mp3 player in it, can I use the ST voice/bluetooth to play the music stored on the phone via the sound system?

And if I can, is the sound quality likely to be OK, or do phones use a reduced sampling rate to save on memory which degrades the quality?

That is a good question and i have no idea to the answer. I would GUESS that it wouldnt work because of the way the system in the car works.

I'll try and do a bit more digging. It can handle a phone call that way, and obviously data transfer between handset and sound system happens then, but that wouldn't need a particularly high bit rate.

Try Aux mode while your phone is connected, then play a track on your phone and see if it comes through the radio.
I doubt it will work though

i have tryed this with my walkman w850i mobile and cant get it to play through the car stereo,thought it would work as it plays through my bluetooth wireless headfones...:(