It's defaulted to 1 for the Sprint mogul in the 2.17 rom
Joint Stereo uses a lot less bandwidth, and for a lot of folks is close enough to "real stereo" - it blends the center channel a bit, so it sends less data overall.
Interesting post over on the htcwiki
http://www.htcwiki.com/page/Tweaks+and+Hacks?t=anon
mprove BT A2DP sound quality
[HKLM/Software/Microsoft/Bluetooth/A2DP/Settings]
Add new dword, Value name=BitPool; Value data=48 (Decimal)
Modify, Value name=UseJointStereo; Value data=0 (Decimal, Originally 1)
For BitPool:
Microsoft suggests the following bitpool values for optimal buffer sizes.
30 - Low audio quality
40 - Medium audio quality
48 - High audio quality
58 - Excellent audio quality
Chose one you like. I think 48 is good enough.
For UseJointStereo: use 0 instead of 1, then you got the real stereo.