Trying to force downloads at 128kbps, using -x --audio-format mp3 --audio-quality 128k

Hi, I'm using the following comment to try to download and convert to a 128kbps MP3:

yt-dlp --ffmpeg-location c:\ffmpeg\bin -x --audio-format mp3 --audio-quality 320k LINK

Unfortunately the MP3 that gets generated seems to have a quite random bitrrate. I'm downloading 3 different audio parts from the same channel/series and one is 96kbps, one is 128kbps and another is 320kbps.

Ideally I'd like them to all be 128kbps, but also if possible, if it's lower (like the 96kbps one) I would prefer not to convert it at all if that's an option?

The files are for my son's Yoto device and their cloud service converts everything to 128kbps anyway so I'd prefer to not go over that if I can help it, thank you :)