Welcome to the forum.
This often happens in Sceneline View Mode, when one has DD 5.1 SS Audio and they have not set up the Project using that Preset. This often happens with AVCHD material, that has the DD 5.1 SS Audio source.
If you have things already setup, then you might want to change to Timeline View Mode, but doing the proper Preset would be better.
PS - in Timeline View Mode, you might have to scroll to see where the Audio portion of the muxed AV file has gone - likely Audio Track 4 or 5.
Many thanks Bill, you got it right on the nose!, it works.
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That is great news, and thank you for reporting your success!