Allegedly, on or about 05 February 2014, Ed Greshko sent:
For that I simply use "pavucontrol". I use that instead of "kmix" since it allows me to place the volume at 153%.
Caution with that... It works well, for dealing with too-quiet audio (audio that never, originally, reached maximum output, can be boosted up to maximum). But as soon as you play something that already was at full level, you're trying to boost it beyond what it can be boosted without heavy distortion, and it sounds awful. Not to mention painful. I've experienced that once too often.
If you have recurring problems with playing very quiet audio, it may be worth finding a player that can do automatic gain adjustment. Boosting low audio up to a better level, and not boosting already loud audio.