I’m attempting to normalize levels in a music compilation, 20-30 songs at a time. The normalization seems to do what I want, making the volume levels sound closer across all the tracks. That works fine. The problem here is, Audacity changes the “artist” field in the ID3 tagging to all the same artist. I can’t find a command that tells Audacity to keep all existing tagging with no changes. Any ideas?
Tags are a long ongoing “weakness”. 
But the existing tags aren’t actually changed unless you over-write the old file. The old file isn’t changed at all unless you overwrite it.
MP3tag can copy from the old to the new.
Or you might want to try MP3Gain which changes something the the MP3 headers to change the volume without re-encoding the MP3
Or, ReplayGain which adds a special volume-change tag, if your player software supports it. Or Apple supports Sound Check which is similar to ReplayGain.
All of those can batch process.
I wanted to hard-code the volume in the tracks, because this compilation will be used by multiple devices and platforms, some of which don’t use ReplayGain.
I didn’t overwrite the originals, I exported new copies of the files in a separate directory. Audacity is grabbing the artist from the last song and filling in that artist and overwriting the artist in all of the other songs.
MP3Gain does that.
Or copy the tags with MP3tag.
MP3Gain (and ReplayGain) also checks the peaks so by default it won’t over-boost into clipping. This means some quiet-sounding songs with high peaks won’t be boosted enough to hit the target loudness but you won’t accidently get clipping. And loud tracks are turned-down so overall it works pretty well.
Does mp3gain work with FLAC or WAV files?
No… I don’t know where I got the idea you were using MP3s!
(MP3tag works on almost everything.)
BUT you can find WaveGain here. I don’t know of anything for FLAC so you’d have to process the WAVs before converting to FLAC (or convert the WAVs to FLAC and then back).
I solved this problem for myself in the following way. I wrote a macro, where the first action is setting the desired volume of the file (Loudness Normalize), and the second action is Exporting it to WAV (choose the format you need). In Audacity settings, make sure to specify the folder for the files generated by the macro. This way, you won’t need to copy anything manually — all tags for each individual file will be preserved in their original form.
P.S. Next tab Tools → Macro Manager → select the desired macro in left field → Files button → select the desired files