Shaky eggs, or "performers" with no ear/rythmn/voice - in this case it's all the same, a person completely oblivious that their contribution is not wanted nor is it appreciated.

In the case that I refer to (& JennyO knows it), I have contemplated a piece of 4x4 thumped repeatedly on the offending head.

sandra (who sings along with the chorus cos she knows she can't hold a tune on her own)