I have a Soundlink Mini 2 and the volume seems a bit hit and miss... Basically as you're listening to a song it seems like it can't quite decide what volume be at as if it's someone choosing between 2 settings. It's quite subtle but when the rest of the audio quality is so good it's kind of annoying.
Could it be some kind of fluctuation in the bluetooth connection or some nearby interference?
That shouldn't be happening - when did you notice this start happening? Have you tried a reset as shown HERE?
What device do you have your speaker connected to? Have you tried connecting to any other devices and if so, is the issue persistent?