Shameless plug but I just re-released my Volume Mixer plugin to the store. It allows you to control the volume of games, programs, and connected Bluetooth sources like phones. I use it all the time since I usually have a podcast or stream going on my phone while playing a game
Yup! The Steam Deck (or Linux really) lets you use it as a Bluetooth speaker. So I connect my phone to the Deck and my AirPods to the deck and can listen to anything that’s playing through my phone and the deck through my headphones.