If Playbar is enabled and you have an audio file linked to a documents (audio file in the documents folder like an MP3 etc), the Playbar will appear on the bottom of the window. It's a darkened area that you can affect playback of the current file by performing gestures on the area.
Here are the commands:
A Single tap will start and stop playback.
A two finger tap - will set a start position for playback.
A two finger long press will clear the start position.
A double-tap will start playback from the start position.
Swipe right and left will jump playback +- 4 secs.
Press and drag either left or right will rewind and fast forward.
A long 1 finger press will jump to location of finger on bar - long finger press in middle will jump to middle of song, at left - from close to the beginning, at rightmost on the bar will jump to a position on the end.