I picked up my One X+ on Friday, set it up and on Saturday paired all of my Bluetooth devices. Exact same problem: through the car's built-in hands-free the phone sounds like is is on max volume, the speakers buzz and hum, (it sounds like if there was an equalizer attached, all levels are at "10") the voice quality is so poor that I dont really want to use it. Called ATT and HTC, tried all the diagnostics they suggested, tried two different One X+ phones, same thing. Tried adjusting the BT volume on the phone and on the various devices (car, Jabra and Jawbone) and no change.
GOOD NEWS This afternoon I spoke to HTC again, they said they are aware of the problem and are working on an update / fix, perhaps as soon as 2-3 weeks. I can live with the problem that long, no need to trade out the phone.