If you put a comparator behind a command block and as command you enter a command (e.g. "playsound mob.pig.say @p" or "tell @p hi"), the comparator will turn on as soon as you activate the command block and the command runs (e.g. song/sound plays). But when the command stops (e.g. sound is over or the text is printed), the comparator doesn't turn off.

I would have expected, that the comparator turns off as soon as the command is ended. So it would be the same behaviour over all. I mean, a command is only true, as long as it runs.

It also seems, that the command block stays "powered". you can destroy the comparator and put a new one down there, it will be immediately powered again.