Robert,
It's a bit hard to diagnose your problem without knowing your signal chain and/or your FX settings. If you have a decent mic going straight into your mixer with no FX, you should get a clean sound. If not, there a couple of things to try:
1) use a different mic. Does it still have the "muffled" sound? If not, then you can assume the problem is with the mic.
2) If so, then it probably has to do with you mixer settings--too low on input? Impedance? Phase?
If you can clarify your signal chain that will help.