Try turning off your tablet and restarting it. If it won't turn off, hold the sleep/wake button until you see the Google logo, or whatever splash screen your device displays first when booting.
If this doesn't work, then look at your user manual and see if there is a reset switch. Usually it's a small pinhole that you'd take something like a paperclip or possibly the tip of a ball point pen to press to reset the device. Again check your user manual to see if the device has this reset switch and where it's located. I won't be responsible if you accidentally stick a paperclip into your microphone's port and ruin the microphone.