Number of actually read frames. Multiply by getNumChannels() to get the number of read samples. When that number is less than frames, it means the stream is done decoding, or that there was a decoding error. If there was an error, isError() is set. Do not call decoding again in that case.
TODO: once this returned less than frames, are we guaranteed we can keep calling that and it returns 0?
Read interleaved double samples in the given buffer outData.