The firstValidByteOfBlock element shall give the local address within the block of the first byte of the first audio sample. The local address range shall be understood to range from byte 0 to byte audioDataBlockSize - 1.
Used only when non-audio data is also included within the audio data block. Use with lastValidByteOfBlock and audioBlockSize elements to describe a region of valid audio samples within the block. If firstValidByteOfBlock is present then the lastValidByteOfBlock and audioDataBlockSize elements shall be required.