For some odd reason, the audio preview on my Ubuntu machine became inaudible. I thought that there was some missing library but I knew that it worked before and had only recently stopped working.
I found the problem in my sound preferences.
When I hovered over a music track, the "Audio Preview" label showed up but the audio levels were near mute. Any attempt to adjust the volume with the mouse proved useless as the label would disappear once the mouse moved off the music track.
The solution that I found was to use the keyboard. By opening the window first, and hovering over the music track, I could use the tab key to move to the slider and adjust the volume.
This solved an annoying problem and I hope it helps someone who has the same problem.
