Did you upgrade your level of Android in that timeframe? I am having the same problem, and I think the sound problem coincided with my upgrade to Android 4.3. I contacted HTC about the problem, but the only thing they suggested ended up wiping my homescreens of all my icons and widgets, and the sound did not get better, so I am wary about trying anything else.
If anyone else has had this problem, and you managed to get the full Boomsound level back on your phone, please let us know how you did it.