To sing or not to sing, but definitely no barking

I don't know what to do. You see, there's this little girl who sits right behind me, and she's been singing Christmas carols for the last few days. She sings them every morning and afternoon. I have no idea what song she's singing, but I know it's a Christmas song because I hear her say "Christmas" and "presents" and "Santa." Everything else is pretty high, squeaky and unintelligible -- and it's driving me insane.

Friday morning, I decided to grin and bare it for as long as I could in the hopes she'd get tired and stop. But she didn't! Fortunately for me, she started barking like a dog for some reason, and that's when I told her to sit there and be quiet for the rest of the trip.

But what do I do NEXT week when she starts singing again? Do I sit there, biting on a bullet to lesson the pain? Or do I squash her "festive self-esteem" by telling her to knock it off so I can drive in peace and quiet?

Oh, the decisions we bus drivers must constantly make! Especially around the holidays!