So I'm currently on an iPhone 4S with the most up to date iOS (8.4.1) and using the most up to date iOS player. My Server also runs the most up to date version (0.9.12.11).
When I try and play music on my iphone, I haven't tested it with other files at the moment, the app crashes after some time. If I play a short song, it will crash after it has finished playing a longer 2h audio clip will make it crash somewhere during playback. The screen is basically black. Sometimes it will return to the Plex Home screen and pressing play on the file again will make it stall and not connect. I then have to force shut Plex and reopen it to work properly.