If you look into the source code of the audio demo, you can see there is an IF statement on line 48. It compares whether
button B is pressed and
music is not playing, when result is true it will call code inside that brach. So to make it work like you need to just remove
jo_is_pad1_key_pressed(JO_KEY_B) && from the IF statement.
I recommend learning some basics of C language before (just simple stuff, IF statements, functions, switch cases, ...), so you can read the code more easily.
EDIT: I forgot, the actual function that starts playing music is
jo_audio_play_cd_track(), you can find what it does in
joEngine docs.