Parenting Conundrum

My kids sing... all the time.  Sometimes an actual tune, often just noises.  But still... I love that they have something in them that they're comfortable and happy enough to let out.

At the same time, when they're doing homework I know darned well that they cannot concentrate on reading, math, etc. when there is a  non-stop lah-lah-lah-do-doo-doo going on.

I don't want to squash their song, but I have to get them to focus on their work.

What's a mom to do????