I just played a game as DJ Sona. I was able to hear her every music. Althought my friend who I was premade told me, that she couldn't hear it. There also wasn't that icon on the left next to Sona, where you could put the music on and off.
tl;dr You can't hear the DJ Sona's music if you are in same team with her.
After quick browsing I didn't find any discussion about this yet so I thought I could make one? I'm sorry if it was already somewhere and I just missed it.
(So sorry for my clumsy English. x_x)