I had the same problem with a Samsung phone. There are two ways of fixing this; First, you can remove the S9 profile from your phone's bluetooth list, and then add it again, going through pairing and entering 0000 on your phone when asked for the code.
Alternatively, you might try pressing the "play" button on the right earpiece on the Motorola S9. This is supposed to kick it into the A2DP stereo mode for music listening.
And get a copy of the manual from the Motorola web site at http://direct.motorola.com/ens/s9/Headset_Home.asp
I found this very handy for checking what the little flashing lights mean. When you get stereo reception, the little LED in the middle of the headset flashes green, not the usual blue and red and purple for voice calls.
In case you didn't know, as a phone headset, with the microphone activated, you only get sound in your left ear, and the light flashes blue. For music, you get both ears, and it flashes green. So if it ain't green, you are in mono mode.