I was able to open the AC3 in Goldwave with no problem. I just dragged and dropped it, and it opened. I can hear bad microphone work. There's a nice clean noise print at the start of the audio, too. I first ran the aggressive pop/click default in Goldwave, and it knocked out most of the bad audio fuzzles. And then I did a noise reduction, still in Goldwave, with the default it pulled up, using the clean noise print already at the header of the file. It cleaned up well, with no artifacts that I could hear.
But the audio was a tad low, so I opened it in
SoundForge, did a normalize curve at 60%.
Converted to AC3 in
TMPGEnc AC3 encoder (Sound Player app).
All in all, this is an easy one.
I can only guess that maybe you've installed some software or codecs that have disrupted how your system is handling AC3 audio? That's what it sounds like.