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.
This icon comes from BoomSound, installed on your HTC device. Android Central forums has a thread about it. This icon pops up when there is sound to play on anything active on the device. Web pages, messages, anything including something with sound.
Quoting from one entry:
The thing about BoomSound that it is set so that it automatically works whenever any sound is played through the built-in speakers.
It never really bothered me, however, it would've been nice if they made it possible for the user to turn-off BoomSound, this way users can choose to use it or not.
So even receiving a phone call or a notification will trigger the BoomSound to work.
Here's a page that describes settings available for it, even if it is describing the M9.