Process priority of music player?
I believe why this is happening is because when your phone goes into standby the processor goes into a state where the processor slows down to save battery life. You could try to see if there is a feature in your mod to turn off this mode. 

I think your phone doesn't go into power saver mode when it is plugged in, try that and see if it works (this solution is less convenient because your phone needs to be plugged in). One last thing I could think of would be trying to kill some services that are hogging cpu. This solution will be more likely to drain your battery if you kill some services that automatically start up again. 

Good luck!