debian/null.ogg is Ogg Vorbis audio file that contains 10ms of silence.
It is used as a stand-in for removed sound effects.  It can be recreated
by installing 'ffmpeg' and running the following command:

ffmpeg -f lavfi -i anullsrc -t 0.01 -c:a libvorbis null.ogg

(10ms was chosen because asking for 1ms seems to create an invalid file.)
