When I'm playing music on my iPad over bluetooth, sometimes when a track ends the next track will start, but there will be no sound. I can see the bar progressing and the time counting up as the songs plays, but the audio is muted. If I press the back arrow to go to the beginning of the song on the lock screen of my iPad, the song will start playing from the beginning and the audio will be fine. I can also open up the Spotify app and use the scrubber to move to another point in the song and that will bring back the audio as well.
I haven't had time to test if this happens after certain songs or if it only occurs on bluetooth, but I'll be testing that when I get the chance. I've already re-installed Spotify on my iPad and re-synced all of the songs from my iTunes library.
Device info: iPad Air 2 running iOS 8.1.3, but it occured on earlier versions as well. Spotify 188.8.131.523. This has been happening since I got my iPad in November. I don't remember it occuring in January, so it's possible it was working perfectly with Spotify 2.2.