YouTube and Spotify both use Opus (Vorbis container) as their main codec, using AAC (MP4/M4A container) as a lower quality fallback.
YouTube also only offers it's Stable Volume feature in Opus, via DRC. The current situation means it cannot be passed through to Sonos and needs to be transcoded first.
Please add Opus, it needs minimal decoding power, is much more modern than either mp3 or aac, and is not patent encumbered.
This has been requested for years now, two examples:
