She has a music note bubble over her head before she says this line. However, that particular emotion bubble is generally used to convey happiness. I needed some way to convey the fact that she was actually singing. While having *signing* in the text box might not be the most professional answer, it was the quickest (read: laziest) one.
I might try delineating it, but I think it's more silly this way. And Toshie is supposed to be silly.