This is caused by Power Amp v2. If you search the Google you'll find official developer's comment about this. The bug is caused by MIUI framework. He said he won't make another version just for MIUI users. Bad news.
Q. Poweramp stops/pauses when in background/when screen is off
This can happen if some Task Killer kills Poweramp in background. Please add Poweramp into exceptions list.
Note, that on Android platform, killing one app can cause other app to be killed as well. For example, killing any app related to media services/android library/audio will most probably kill everything related to these services. I.e. killing stock player will kill Poweramp as well.
(On some Motorola phones, the stock Task Manager has Auto End list instead, not exceptions).
Poweramp 2.x: Note, that some (custom + some stock Huawei) ROMs have issues/bugs which kills OpenGL apps (i.e. Poweramp 2.0) when screen is off. This can't be fixed from app side, please report this bug to ROM developers.
so we're out of luck for the moment..help us plz...