I ran into a similar issue when I tried to generate a podcast RSS feed from a website whose built-in feed didn't go back far enough. I was trying to do HTTP ran

I ran into a similar issue when I tried to generate a podcast RSS feed from a website whose built-in feed didn't go back far enough. I was trying to do HTTP range requests on the mp3 files to save bandwidth and just fetch their metadata. Sure enough, mostly no duration and if the encoder did put it in a custom field it was usually different than what VLC says.