Just a PSA and a hope for a fix – Audacity doesn’t natively support FFmpeg files (like mp4a files), but the error message you get when you try to open those files sends you to this Audacity support page for a plugin to allow import/export:
Unfortunately, the “recommended installer” link (starting with lame.buanzo…) brings you to a nondescript download page which installs “PC App Store” on your computer, a known adware/malware app. It’s probably a case of a webpage takeover (I’m sure it was a helpful link when the Audacity support page was written), but it’s now a threat to anyone following prompts directly from the Audacity app.
I added feedback to the support page with the bad link, but I have no idea whether the Audacity support team will see it there – so I’m posting here in the hope that someone will see it and remove the link and no one else will have to go through this nightmare.