al_drain_audio_stream man page
al_drain_audio_stream — Allegro 5 API
#include <allegro5/allegro_audio.h> void al_drain_audio_stream(ALLEGRO_AUDIO_STREAM *stream)
You should call this to finalise an audio stream that you will no longer be feeding, to wait for all pending buffers to finish playing. The stream's playing state will change to false.