While transcribing, I wanted to use a Sony FS-85 foot control unit to pause and rewind the audio while I typed.

When plugged into linux it appears as a keyboard and gives various key-press codes when you hit the left or right petal. (The right side gives F10 when you press down, F11 when you release, etc). I was able to make this control Mplayer by adding a ~/.mplayer/input.conf file with the following lines in it:

#
# .mplayer/input.conf file appropriate for using the Sony FS-85
F11 pause
5   speed_set 1.0
;       seek -5

One of the key codes the left side was sending corresponded to Mplayer's speed_mult command, so I had to throw in a speed_set 1.0 command to counteract that. (The correct way to solve this is to figure out what key it is sending and override that, but I'm lazy and this works.)  With this file, pressing the right side pauses the audio (on the press, releasing it does nothing, you must press again to start playback again) and pressing the left side skips back five seconds.

The only problem with this setup is that focus must be in the mplayer window for the keystrokes to take effect, which precludes you from typing in a different window. I solved this problem by using two computers (one for playback, one for transcription).