There seems to be a problem with Famitracker and sleep mode.
I'm running windows 7 Ultimate, and I've set the computer to go to sleep after 15 minutes. If Famitracker is started, the computer never goes to sleep. I've tested with various other applications, and they all seem to not be "in the way" of sleep mode. If I put the computer in sleep mode manually things work fine.
That's probably because your audio drivers is refusing sleep mode. Famitracker is using a direct sound buffer that is always active even when there is no sound.