I sing to Olivea all the time and while she doesn't cheer, laugh or smile, she at least stares transfixed at my face (much like she does at the TV, the ceiling fan, and the screw holding the computer desk together). When Mat sings to her though she makes these horribly sad faces. Somebody kicked her puppy kind of faces. Perfect frowns grace her face, and she looks like she's going to burst into tears at any given second. So of course he sings to her often, and I watch and laugh. It's just too funny, and frankly she's a little young to be scarred* for life. You might want to turn the volume down, or you'll be making some frowny faces too.
*I checked the spelling of this by typing it into google. Not a good idea. Some truly horrible images came up. I don't recommend it.