Add mode activated and deactivated sounds

* Remove test "activ.wav" file
* Add mode activated and deactivated audio files
* Add two new methods to play these two sounds without having to pass in
  `MODE_ACTIVATED` or `MODE_DEACTIVATED` from outside the module
* Make `play_audio()` private

The 'rodio' crate supports WAV, FLAC, MP3, and Vorbis file types. Tried
using OGG and MP3 versions of the audio, which sound effectively the
same played in an audio player, and have much better compression, but
'rodio' kept playing them choppily, in a weird staccato. Maybe the
sample rate wasn't right? No idea what the problem was. Didn't really
have the same problem with the WAV files. Unfortunate because the OGG
files were only 16K, and the MP3 files were 32K, while the WAV files are
348K. Would much rather have smaller size files, but if the sounds play
incorrectly, there's no easy way around it.

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