So everyone understands the occasional problem of "missing" audio......
A typical AES
encoder such as the Dolby DP569
when set to stereo expects the following:
Left/Right AES audio to input 1 of the encoder.
However, the AES encoder when set to Dolby 5.1 expects the following:
Left Front/Right Front AES audio to input 1 of the encoder.
Center/Sub AES audio to input 2 of the encoder.
Left Rear/Right Rear AES audio to input 3 of the encoder.
As you can see, if the encoder doesn't switch the mode to 5.1 when the signal changes 5.1, you would only hear the Left Front and Right Front channels which likely only contain extraneous noise and not dialog.
The change between modes (presets) is usually triggered by a GPI (contact closure). This is a typical failure point. As you can see on the Dolby DP569 there are buttons on the front to manually change the preset however, if the master control operator isn't monitoring the audio or is only watching a VU meter
they may not know the encoder didn't switch. Additionally, some broadcast facilities may not allow their master control ops to manually change the encoder instead requiring a call to the engineer to do so. This may also result in a delay for the needed switch.