You've probably sorted this by now, but I am intrigued - a USB mic would require that the mic has an analogue to digital converter inside it as USB is a digital connection to the computer. Hence, it must have some kind of levelling inside (probably automatic like old tape recorders)...so in summary, unless there is a switch/doobery on the mic itself I'm not sure what you could do (so all round pretty helpful!)
What model microphone is it?
Cheers
K