What is this circular icon that keeps appearing/dissapearing? This keeps appearing/dissapearing on my icon bar, and I can't find any reference to it.  Can anyone shed some light on this mystery?
This means that the audio you are playing at this moment is supported by the "DolbyAudio" sound-enhancement. Seems like you are using a HTC ONE M9 then - it is a replacement for the BeatsAudio engine to enhance your music.

I found the explanation on a german site. Credits go to them.