ヘルプに全部書いてるな
WebプレイヤーだけAACなんだな
https://support.spotify.com/jp/using_spotify/system_settings/high-quality-streaming/


https://i.imgur.com/QcG1PRR.png
https://i.imgur.com/lEYs8Si.png
https://i.imgur.com/rsuTYVY.png