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