Is it normal that the rear speakers from my 7.1 surround sound system are outputting sound even though the audio content played is coming from a 5.1 source, even when Auto Format Decode is selected?

Yes, the system is designed to enhance your sound experience when a channel for the 7.1 surround sound rear speakers (Surround Back Channel) is available.

If a 7.1 surround sound system is connected to your Sony AV Receiver, and it is playing 5.1 audio content, your Sony AV Receiver will copy the audio to the rear speakers of the 7.1 system in order for you to enjoy the full performance of your 7.1 surround sound.

IMPORTANT: This will also happen even when the Auto Format Decode (AFD) feature is selected.

The sound level of the rear speakers will be automatically adjusted according to your configuration, maintaining an adequate sound balance between all of your speakers.

NOTE: This feature will not apply when audio in Direct-Stream-Digital (DSD) format is being played, for example if you are listening to a Super Audio CD (SACD).