I’m using the Tritton AX720’s and would have run into the same problem…Luckily Razer had thought of that…from their website:
"Q: Does the Razer Onza support microphones that connect to the controller?
A: The Razer Onza has a standard 2.5mm microphone jack to which any microphone using this jack can connect. Of course the best choice is the new Razer Chimaera™ 5.1 Wireless Headset for Xbox 360."
Link: http://www2.razerzone.com/onza/faq
You can “mouse over” the “X” where you would normally plug in your headset mic.