Have you ever watched a video on Youtube using a faster play speed (1.25x, 1.5x,..) and the sound seemed “the same but faster”. Ever did the same (using [ and ] keys) in mplayer, and all the voices were higher (high-pitched), and hard to listen and understand? There’s a filter in mplayer that fixes that and it’s called scaletempo.
mplayer -af scaletempo /foo/video.mp4
Now you can change the playback speed without the annoying pitch changes!