This function must flush the audio decoder, i.e. it must reset itself to a clean state
so that DecodeAudioFrame() can be fed a fresh packet without
the risk of confusing the decoder because it was expecting a different packet. This function
is called by Hollywood before seeking the audio stream.
Depending on how flexible your decoder is, this function may not need to do anything
at all.
This function must be implemented in a thread-safe way.