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

Closes #419 (closed)