Cobalt fails to download YouTube audio format 251

[overcast07]

bug description

Cobalt does not choose YouTube audio format 251 (best quality opus), even when selecting opus as the preferred format. It transcodes from aac instead.

Someone mentioned this on Discord a month ago so this has been happening for a while. https://discord.com/channels/1049706293700591677/1049740077460377660/1368652883846889563

reproduction steps

1. Find a YouTube video and confirm that it has audio format 251 using yt-dlp (e.g. yt-dlp -F "https://www.youtube.com/watch?v=VGBrG88QW6E")
2. Download this audio from the Cobalt website in audio-only mode using any settings. The aac/m4a audio will be used as the source audio regardless of the options

screenshots

No response

links

platform information

All

additional context

This is relevant because the aac audio on YouTube is actually worse in a very noticeable way: all frequencies above 16 kHz are removed. Opus goes up to 20 kHz.

This is a regression, as previously I recommended Cobalt to people specifically because it didn't behave like generic YouTube downloader websites and would generate an MP3 by transcoding from opus instead of aac.