Hmmm....The Mbox is digidesign hardware, not m-audio.
Take a look at the mbox hardware. It has a little button with some led's to the right of it for mic, line, and inst. Use the button to switch between (if your mic is plugged into the mbox then you should have mic selected). The Mbox 2 has a similar button with led's for DI or mic. If your source on your mbox is set to mic already, then the problem is either with the mic itself (unlikely but if you have a second mic of the same kind test it anyway), with the mic cord (unlikely but test it anyway), or with the Mbox itself. If you figure out it is a problem with your Mbox you will probably have to get in touch with Digidesign to get it fixed.
Good luck!