I have an important bug to tell you. Whenever I import an .ac3 audio file which is taken from a DVD video, the audio becomes very quiet whenever there’s something loud. Then after a while when there’s been enough silence/quiet parts, the audio becomes automatically loud again. I’ve broken too many audios with this, and the only way I’ve fixed this is by using Foobar2000->.wav 32bit Float conversion. Then the audio has equal loudness after importing it in audacity. Interestingly this only happens with some .ac3 files, not all of them, even though the formats are the same and the source is the same = a dvd.
This is an extremely stupid issue, I really, really hope you fix this so others doesn’t have to face the problem in their projects.
bottom=original, top=compressed/broken by audacity, no other software i’ve tried does this. so much time wasted…
If you have a workaround with foobar, use it. 
Strange problem… It’s very-likely something in the FFmpeg CODEC which the Audacity developers don’t control. FFmpeg for Audacity was updated recently to 5.0.0 so you might try upgrading. (Or maybe 5.0.0 is the problem…?)
I have had AC3 files with constant low-volume throughout and I think that’s related to metadata that’s being ignored by FFmpeg, or something like that. But in that case you can simply Amplify. Plus, they do tend to leave “headroom” on DVDs and possibly extra headroom for downmixing of surround tracks.
bottom=original, top=compressed/broken by audacity,
There is DRC (dynamic range compression) metadata on a DVDs and with an AVR you can switch it on & off. Sometimes it’s called “midnight mode” so you can listen quietly and still hear the dialog without getting blasted by loud effects. But I’m not sure if you’re hearing normal compression.
I assume FFmpeg has a way of switching it on or off but Audacity can’t access those options.
Yup, i use foobar, but there are some days i forget and waste time working on a broken audio.
And i just now tried ffmpeg v5.0.0, but nothing changed sadly.
There’s a relevant discussion on this FFmpeg message board: [FFmpeg-devel] lavc/ac3dec: disable DRC by default - Patchwork