MMB - Mumble Mumble Bumble? MusicMatch Box?
Edit - OOPS! you mean Middle Mouse Button... hahaha it was when you said mute/unmute..you can't mute with the middle mouse, it is a scroller, or can be used in a Word document to click as another type of scrolling action, click the MMB then move the mouse up and down to scroll the document.
Go to Control Panel > Mouse and change the action for the MMB. The MMB should be a wheel and not a button, you can still clcick the wheel, it's still a button, it just looks like a wheel.
You can't use those old 3 button mouse/s, they are only usb 1.1 at most.
Edit - Actually, the MMB click option does scroll my internet page.