Add ambient background sounds to zen player

[jwalthour]

possible options:
nature:
https://freesound.org/people/rendersound2214/sounds/397911/
https://freesound.org/people/reinsamba/sounds/18765/
https://freesound.org/people/Arctura/sounds/39829/
https://freesound.org/people/RHumphries/sounds/2523/
coffee shop:
https://freesound.org/people/Trainscough5407/sounds/404478/
https://freesound.org/people/AwenAudio/sounds/403064/
https://freesound.org/people/kfosse13/sounds/423689/
https://freesound.org/people/soundslikewillem/sounds/264347/
(has a child unfortunately) https://freesound.org/people/CharleneZ/sounds/326371/
https://freesound.org/people/justingregoire/sounds/205168/

[jwalthour]

Also make them configurable by dropdown.

Not shown above: might be great to have a rainstorm and/or ocean waves.

[jwalthour]

It turns out these files are several tens of megabytes as uncompressed .wav files. So in order to support this, we need mp3 decoding support.