I don't know why it happens with only some videos, but it's annoying. Also, why is there even an option to download audio in 256 kbps or 320 kbps? To the best of my knowledge, YouTube has never offered audio above 192 kbps, and even 192 kbps isn't common, and I think it isn't even an upload option anymore. 128 kbps is more common. That's barely acceptable on YouTube's end, but I deal with it since sometimes I can only find certain songs on YouTube. But I don't want to be downloading converted audio, because that results in quality loss.