emm, I think you both may be misunderstanding what the shift shift space does to launchpad... don't get me wrong, I do that aswell when things go buggy outa desperation. But what is actually needed is a command line kill of mplayer which should be pretty easy for anyone to setup I think ?
In my mind launchpad is just a shell.. basically tells the kernel what to do, I dont know if you have ever tried killing explorer.exe on a windows machine, to most users it looks like the pc is dead, if you load up explorer.exe again tho all of your processes(and therefor apps) are still running. I have never been a *nix geek and I know even less about the kindles version, but from what I have seen it seems to be similar. In summery I guess what I am saying is much like any shell where you can't actively kill a bugged prosess, the only option is a hard reboot
I do it most of the time after listening to mplayer, only way I know it wont be running in the background
http://linux.about.com/library/cmd/blcmdl_kill.htm
hmm, so I guess we need to know the id which makes sense but heh can get mixed up so hmm,,, would basically want a line that asks for for the mplayers id / psid / whatever, and to give it back a result that you can put into a variable to kill that process ?
surely we can do this together without annoying Smarter
http://linux.about.com/library/cmd/blcmdl8_pidof.htm
just need to combine the two