Interesting that even remuxing does not fix this. I will try recoding, given that flac is lossless.

I fixed them by running:

ffmpeg -i 01_-_Gute_Nacht_\(CK_2946-2\,_ES_383\).flac -map 0 -codec flac output.flac

Just copying the stream didn't help. But flac is lossless, so it is OK to recode.

For one file I had to do:

ffmpeg -i HPR-h4ck3rm1k3-2013-09-22-Whatsinmybag2.flac -err_detect +ignore_err -map 0 -codec flac output.flac

Okay that's just a workaround then. Maybe it doesn't even fix it since the duration is already set properly locally. Moreover, it would not fix this with bulk imports or imports via video2commons once and if such becomes possible.

No, duration is not set correct locally. For files I fixed above the following did not work:

ffmpeg -i 01_-_Gute_Nacht_\(CK_2946-2\,_ES_383\).flac -map 0 -codec copy output.flac

So this just remuxes the file. But the result was still 0 length. I had to recode them. So I do not think "locally" works. Maybe for some tools. But not for all. And it seems not for libraries used by Mediawiki.

I was talking about the link I posted. Let me know if I should rather open a new issue about it. ffmpeg and all other tools show the proper duration locally. I'll add the link and the info to the issue description in case people don't see the comment.

Yes, https://commons.wikimedia.org/wiki/File:Raspberrymusic_-_Aliens_(trailer_music;_cinematic_epic_electronic_classical_music).flac seems like a different problem. So maybe another issue would be better. Because that one is 0:0 in Commons but is not in ffmpeg. The files I fixed were 0:0 in ffmpeg as well.

Here is a file with wrong duration (much longer than actual). Maybe it's a related problem, e.g. it may improperly read the duration from some metadata.