I like mplayer as a very versatile video player, just the documentation is a bit lacking at times. This is my scratchpad of options I've found useful over the years:
To keep the pitch on your video if you play it faster/slower - Very useful to speed up slow lectures/talks and still understand what's being said.
Use [ or ] to increase/decrease video playback speed.
Increase the volume a bit, useful to boost audio over the standard maximum.
-vm -adapter 2
Play fullscreen on yoursecondvideo output.
Play without audio.
If you know any other interesting options, please share them below..