Our mu'sucks but not for long

The story:

This was embarrassing, but not for me. This is how my cousin heard it (and actually believed that that's what it was for god knows what reason) for the longest time until he actually sang the lyrics that way. I asked him what the hell "mu" was and he said, "You know, as in short for 'music'." We were rolling on the floor by then. Now all he can say when we bring it up and tease him about it is, "Shut up!" The age is his age.