I'm testing some settings on a phone, and unfortunately the only control I have is through adb right now. My issue is that while I can start playback of a music file through the following command, I have no way of stopping it so I can perform the next iteration.
adb shell am start -a android.intent.action.View -d "file:///sdcard/Music/mediatest.mp3" -n "com.android.music/.MediaPlaybackActivity"
I don't know if it's possible to end the playback using another adb command, but it doesn't look like it to me - I see there are no Intents that seem to perform that function, nor any activities that are made to stop the media playback.