Well, with all of that eliminated, I'd say it has to be something with you connection, kinda going out on a limb here but try resetting your router. If that still has no effect maybe it's further down the line, possibly Xbox Live is having voice chat issues? How long have you been having this problem with your mic?

If it was playing back your voice with out cutting out when you were sending a message then it sounds like a connection issue. If you are directly connected to your router (via wire) then reset your router. If problem continues then it your connection. If your using a wireless connection to connect to your wireless router or if you have multiple machines connecting to your router, then it who ever else is on your router is draining the bandwidth, which in turn is making you lag. Make sure you have a password on your router so random strangers aren't using your internet. Make your password WPA2 as that encryption is a lot harder to hack than WEP encryption. It could also be that you have an older router. Google the router you have and find out. If it is an older router I would but a newer version. I hope I helped out! Good luck!