[popcornmix]

Not sure about decreasing the nice level in OpenELEC, though as everything runs as root I don't see why you shouldn't be able to do that.

My suspicion was right that OpenELEC doesn't handle "above normal" thread priorities. I've added this commit to piaudio_sink:
https://github.com/xbmc/xbmc/pull/3840

Note: this will affect omxplayer, which runs audio thread above normal priority, as well as the AE Sink,
so it may be worth testing if hard streams play better or worse than before with this change.