bool al_attach_mixer_to_mixer(ALLEGRO_MIXER *stream, ALLEGRO_MIXER *mixer)
Attaches the mixer passed as the first argument onto the mixer passed as the second argument. The first mixer (that is going to be attached) must not already be attached to anything. Both mixers must use the same frequency and audio depth.
Returns true on success, false on error.
It is invalid to attach a mixer to itself.