Heja!

Does anybody know if you can use the mic that comes with the 360 to sing in Rock Band?

I doubt it, the quality of those mics are really low.  Why don't you just try it and see if it works?

You can if you want. It will totally work. Just not the best way to do it.

I think it is possible to get it to work, but it involves some tinkering with settings and stuff.

I couldyn't test it because i didn't have the game :D

Bill, what is the best way to do it then, besides buying the Instrument box that costs a lot?

It does work.  You can even tap the mic to clap but I don't recommend it.  Use the A button to clap instead.