Dark Echo [official website] is one of the most interesting examples of audio-as-gameplay that I've come across. In fact, if you're not playing with headphones then you're doing it wrong.
The idea is that you're trapped in darkness and can use only sound to guide yourself through the environment. The result is an incredibly weird, minimalist project made up of footsteps, echo-location and abstract geometry of echoing sound, and couched in a plotline about a horrifying evil that devours sound and souls.
Dark Echo is out on Steam with a launch price of £1.33.
Ludum Dare fans might recognise it by its earlier incarnation, a little game made in 48 hours called You Must Escape which at the time got to second place of 1600 entries. Dark Echo is a much heftier game, however. The entire thing is 80-levels big, puzzle-heavy and exploration-friendly. It released on mobile a couple of months back, and has only just launched on Steam, and it's a beaut'. Have a look at the trailer: