Microphone mutes by itself every few seconds

This happens now whether I'm using a headset or the built-in microphone. If I keep my finger on the mute button, I can manage a skype conversation by repeatedly unmuting the on-board mic with the physical button above the keyboard; the Sennheiser mike is working, but I can't get the computer to register its output anymore at all.

I've spoken to Sennheiser tech support; the headset appears to be working correctly and properly set up.

All these things worked perfectly until the problem arose.

W7 Ultimate, Lenovo W520. Sennheiser SD Pro 2 headset (and a wired back-up headset).

Help! And all your help is appreciated in advance.

Read other threads; ran malwarebytes & rebooted. Didn't solve the problem.
Ran W7 troubleshooting specifically for the headset - didn't find anything.

When I go to the windows settings for "recording devices," I get no sound at all from the headphones (although they're working, the mic is sending sound to the headset speakers), but I do get sound registering from the internal mic. I'll keep working on it and update...

Fixed by (i) reboot, not sure whether malwarebytes was relevant; and (ii) updating the firmware for the Sennheiser.