Convert notification sound file to Windows-compatible WAV

Conversion has been conducted using ffmpeg. The audio codec has been
explicitely set to pcm_s16le although that is also the default codec
ffmpeg choses for wav output files when I do not specify any codec. Just
added this if this changes in the future and we want to convert more
files to the same format.

ffmpeg -i notification.wav -acodec pcm_s16le output.wav