external getf : t -> [< `pitch
                     |  `gain
                     |  `min_gain
                     |  `max_gain
                     |  `max_distance
                     |  `rolloff_factor
                     |  `cone_outer_gain
                     |  `cone_inner_angle
                     |  `cone_outer_angle
                     |  `reference_distance ] -> float = "al_source_getf"