Thanks again for the help & suggestions.
All my file failures are the same source video & audio quality but different files (2k HD video & 48k 320kbps audio). The audio attached to the source video has been unlinked from video, deleted and the source 320kbps audio added.
One of my successes and failures are from exporting the same file in 2 parts. I got the first part exported, properties show clean 320kbps output .avi file. I changed nothing without closing the program or changing settings & the 2nd part output .avi file shows properties as 0kbps audio bitrate. The parts have the same source audio and video. 1 success & 1 failure.
Upgraded to the version you linked, v16. This took away all my export file privileges as it turned the free version into the demo version.
I rolled back further to the version I was using to v10.12 and giving this a go now. 1st attempt set the MP3 bitrate to 48k 320kbps. File saved showing audio bitrate as 256kbps in properties. Which is better than seeing zero there.
Attempted to do the same file again immediately after previous one. Set MP3 bitrate to 48k 256kbps. Output .avi file showed audio bitrate as 224kbps.
Tried a final time setting MP3 bitrate to 48k 320kbps twice before exporting. Output .avi file was showing audio bitrate as 256kbps.
Went to a site called LO4D or something & got v16.46 of the free version.
Tried a few sample files only a few seconds long from the same source video & audio.
First try got one saved at 256kbps, second try 320kbps woohoo! 3rd to 10th tries all 0kbps. Noooooo. All from the same source files.