Allow for multiple audio files: different translations

[twitwi]

It would be nice to switch between languages.

[twitwi]

Big question: should they be sync'ed? or do we have a descriptor per language?
The second is more flexible...

[twitwi]

In the end, maybe we just want to (over)load the timing file from an url parameter, e.g., "....pitch/#audio=faster.ogg&timings=faster-timings.json"

[twitwi]

See #5 as it may be wiser to implement #5 before #4 (this one).