How about, "I think you should sing solo -- so low no one can hear you!"
For the male who can't, there's, "I think you should sing tenor -- ten or twenty miles away!"
(It occurs to me that your chosen screen name is a bit musical so maybe those are old jokes to you. If so, sorry!)
More seriously, if she's doing it while people are trying to get work done, a talk with the boss about distracting behavior might help.