DJ Sona: Controlling Music In New Client

https://support.riotgames.com/hc/en-us/articles/203799564-DJ-Sona-FAQ#h1q6

HOW CAN HER TEAMMATES HEAR THE SKIN? Her teammates (not enemies) can opt in to hearing by pressing a button located next to Sona’s champion portrait on the scoreboard. This is a toggle, so the button can be used to turn on and off the music.

HOW DO I TURN OFF THE ELECTRONIC MUSIC WHEN PLAYING WITH THE SKIN? You can turn off Sona's music by going into your Sound Options and turning down the Music Volume.

you don't need to because there is no DJ Sona music, only while you're in-game [zombie-brand-facepalm]

The client and the game are two separate things. The client has no impact on the in-game functionality of DJ Sona.