With a PCM track, there is no way for the receiver to know if the audio contains matrixed surround information (i.e. Dolby Surround) or is just plain stereo. You need to press the button yourself.

A Dolby Digital (AC3) stream can carry a flag to indicate the format of 2/0 encoded programs. Decoders can use this to drive a Dolby Surround indicator, or it can be used to automatically control the Dolby Surround Pro Logic decoder. Decoders may ignore and/or override this flag, so the user has final control over the stereo or Dolby Surround listening modes.