VLC can speed video or audio up/down on the fly, preserving audio pitch. I'm using it with audiobooks and old movies, and also occasionally (without pitch lock) to turn hard house into sweet groovy house or breakcore into playful idm, as suggested by RDJ himself—see https://youtube.com/watch?v=5yBvP3616Wc and https://youtube.com/watch?v=aWqf17mUyoQ

Btw, for some reason ffmpeg doesn't do well with changing speed of music—the result sounds poor compared to VLC. I found that `sox` gives better quality.