Yes, SleepAnalyser knows how long the song goes and will disable the button at the end. Currently there is no feedback from the player. So if it (mplayer) crashed, SleepAnalyser has no way of figuring it out.

I just got a new idea of how we could use gstreamer; instead including it into SleepAnalyser (which made it malfunctioning/crashing) I could run it as a separate process. With a pipe it would be possible for them to communicate. I have to think a bit deeper into it, so please be patient.