`play-sound-file', no mp3 support, sets global volume w/o reset

[Emanuel Berg]

`play-sound-file' doesn't play mp3 files, but it could, as the
MP3 patent expired in 2017. (So between 6y 8m 4d and 7y 8m 3d ago.)

But you can easily convert convert an .mp3 file into an .au
file with

  $ ffmpeg -i src.mp3 dst.au

But play-sound-file has another problem, namely, if you do e.g.

  (play-sound-file (file-name-concat bad-dir "data" "up.au") 0.4)

it sets the global volume to 40% and worse, after it is done,
it isn't reset.

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