Where exactly did the clip originate?
The Copy/Paste operation has often caused issues. This goes back to at least PrPro 2.0. Sometimes, the Audio will come in and work fine. Sometimes, it will not come in at all, and in a few instances, it will work for awhile, and then just go "flatline." I encountered this with a Render, though it had had worked perfectly for days, and for many Renders before - then flatline! The Match Frame (M key with the Audio Selected, and the CTI over it) worked perfectly, and restored the Audio.
Why Copy/Paste is so intermittent is a mystery to me. Why a Render (about the 20th for that instance of mine) "broke" things is unknown, but it has happened to enough others, that something must go wrong.
Close Premiere. Navigate to wherever that clip is stored on the drive. Trash the CFA files there that have the same (similar) name as the problematic MPEG file. Restart Premiere and reopen your project. Let the audio be reconformed, and try your export again.