Playback class used for resampled [AudioStream]s. Playback class used to mix an [AudioStream]'s audio samples to [method AudioServer.get_mix_rate] using cubic interpolation. Returns an [AudioStream]'s sample rate, in Hz. Used to perform resampling. Called by [method begin_resample] to mix an [AudioStream] to [method AudioServer.get_mix_rate]. Uses [method _get_stream_sampling_rate] as the source sample rate. Returns the number of mixed frames. Called when an [AudioStream] is played. Clears the cubic interpolation history and starts mixing by calling [method _mix_resampled].