Have you ever played an online game called "Phonopath?" If you're good with audio manipulations, you might like it.

It's this little game where you have these audio clips that are in itself puzzles. There's a passcode hidden in the audio in various ways--and I mean VERY many different ways, and you're supposed to figure it out. Some of them are obvious. Others may seem obvious at first, and then they get tricky. Others are time consuming. Others... you don't know where to start. I spent the last four hours with it. You can use Audacity as the sound editing system. Do know that these puzzles are for people familiar with audio manipulation. It's so fascinating :D