How to control the iphone’s volume while connected to airplay?

Hello 👋

I have a Sonos system at home. When connected via Airplay to the Sonos, I can’t change my iPhone’s volume.

i tried the volume up and down buttons and the volume panel on the control center.

but both will only change my Sonos volume, not the iPhone’s own volume.

So when using Duolingo or listening to WhatsApp audio message, I can’t change the iPhone’s volume.

anyone knows how to change my iPhone’s volume and not the AirPlay-connected Sonos one ?

thanks 💜