[TheFluff]

v turns subtitles on and off, b and j goes through different sub tracks (if there's more than one). However, MPlayer has no hotkey for switching audio tracks - you have to set the language or the audio track ID on the command line.
Example:

mplayer -alang jpn,eng some_video_file.mkv

This will choose the japanese audio track if there is one, but if there isn't, choose the english one, and if there isn't an english one, fall back on the default. Unfortunately, most fansubbers don't care about muxing enough to set proper language tags, so you might have to experiment a bit and use the track ID's instead (-aid 0, -aid 1 etc. etc.).