The music file has to be A-law or U-law or G.711 at 8 khz, 8 Bits, Mono. To do this you can either use sox or ffmpeg. To use sox:

sox -V input_file.wav -A -c 1 -r 8k -1 output.wav```
Update: Seems some of these options are depreciated, if you run sox with the previous options you get:

sox WARN sox: Option -A' is deprecated, use -e a-law' instead. sox WARN sox: Option -1' is deprecated, use -b 8' instead.

So as it says use:

sox -V input.mp3 -e a-law -c 1 -r 8k -b 8 output.wav``` You can test this by using:

play output.wav```
Or by using ffmpeg:

ffmpeg -i input_file.wav -acodec pcm_alaw -ar 8000 -ab 8k -sample_fmt s8 -ac 1 output.wav``` This will show you infomation on the resulting file:

ffprobe -show_streams output.wav```