why aren't my rear surround speakers very loud?
Why bother using surround for music anyway? Music is recorded in two channel (left/right) stereo, so using surround for it just results in the speakers duplicating channels and you getting a bunch of crosstalk.
If listening to music, set the speakers to 2.1 for best listening.
Anyway, as for your problem, I'm not sure. My guess would be the driver is not being told to send a lot of volume to your rear satellites because the music file is not meant for surround, as mentioned...