Man pages matching name al_get_audio_stream_length_secs in section 3alleg5:

al_get_audio_stream_length_secs.3alleg5.en	debian 10 / allegro5-doc
al_get_audio_stream_length_secs.3alleg5.en	debian 11 / allegro5-doc
al_get_audio_stream_length_secs.3alleg5.en	debian 12 / allegro5-doc