Thanks guys for the tip on update, I checked my 820 today and there was indeed an update available, which I took and seems to have resolved the voice quality issues. After downloading the update (Wi-Fi connection recommended btw) apart from the MS updates there was one Nokia ‘Quality Improvement’ update 
Firmware revision: 1232.5951.1249.1021
Can't see that any case would have an effect on the Microphone? If you remove the Back Cover Battery Case the Microphone is still embeded in the housing of the phone just under the screen glass! For me this is an Hardware issue with so many people reporting this problem? Seems NOKIA just want's peoples money aren't interested in a fix? If more people spread the word about this problem and people stop buying the phone, then may be NOKIA might decide to admit there's a problem and find a fix. Personally I'm getting rid as soon as and going back to Samsung! Biggest mistake I've made buying this piece of **bleep**!!!!
Hi everyone,
Sorry to hear that you're having issues with your device specifically with the microphone. I know some of you might read this on the previous threads or conversations but then again, if the phone is not working fine, be it a software or a hardware issue, it'll will still be best to try troubleshooting the phone. Here are some steps to try to fully isolate the issue:
As per adrianhughes, If you are able to confirm that it is not a network issue by using another phone to call same person and if they can still hear you clearly, then may be issue with the phone itself.
If you are using a headset while in a call, you can check the following:
- Check that you are using a compatible headset, e.g. the headset included in the product package.
- Check that the headset is properly plugged in your phone. Be sure to press the headset to the bottom of the connector.
- Check that you are speaking to the microphone. If your headset does not include a microphone, you need to use the phone microphone. Keep the phone microphone close to you when speaking if there is no microphone in your headset.
- Check that you have not muted your outgoing audio. E.g. on the touch screen devices, check that the Mute / Unmute toggle switch on the screen is in proper state (showing Mute command when outgoing audio is enabled). If you have by mistake muted the outgoing audio, tap the Unmute toggle switch to enable your voice to be transmitted.
If there's no headset connected, you have to make sure that there's nothing blocking the device's microphone.
Once identified that the issue is still there, you can proceed with these workarounds.
Press and hold both the Volume down and the Power keys for 15 seconds. This should get your phone to restart properly again. If doing this still doesn't resolve the issue, you have to make sure that the phone runs its latest software. To check for available software update, go to Settings, Phone Updates, Check for updates.
If none of the above troubleshooting steps work, you have to consider doing a reset.
Warning: Resetting your phone will return it to the same state it was in the first time you turned it on. All your personal content in the phone, including text messages and apps that you been purchased and downloaded (may be re-downloaded again) will be erased, and the phone will be restore to the factory settings.
Here are two options to follow, depending on whether you can access your phone's Settings screen or not.
Option 1: About menu
- Use this option if you can access your phone's settings.
- This option warns about data loss.
On the Start screen, swipe left, select Settings > About > Reset your phone.
Option 2: Hardware key combination
Step 1
Make sure the charger is disconnected.
1. Switch the power Off.
2. Press and hold the Volume down and connect the charger, if succeed an exclamation mark (!) is shown on the screen.
Step 2
Then input Keys in the following order:
1. Volume up
2. Volume down
3. Power
4. Volume down
Step 3
The phone will reset and boot up automatically. Please note that this may take several minutes.
Now, if issue still persists even after doing all the possible solutions or once the issue has been identified to be a hardware problem then, it'll be best to send the phone in to your local Nokia Care Point for further assessment.
Refer to this link to find the nearest Nokia Care Point: http://www.nokia.com/global/support/locations/
Hope these information help. 
Regards,
Having read the last post, I switched my phone off! Then held the volume down key and power key for 15 seconds. Phone restared as normal. Checked to see if any updates were available and one was. Downloaded the update and my Firmware has now jumped from 1232.5957.1308.0002 to 1232.5957.1308.0023 Quite a big jump? Still have to test phone to see if there is any difference. I'll post back if there is, or if any other user does this and finds that it does make a difference then please post it on the site. Here's hoping. 
Hi periwinkle28,
I have done this already many times, and even had it shipped to Nokia to get it worked on. And even after all this it still happens. It is very inconsistant, but seem to happen more when calling into call centers. Hopefully their is something else I can do, or give further info to get this issue resolved. 
I bought this protection case cover for my 820 from ebay and after that I started to have problems too that others couldn't hear me. So the problem is, that the microfon is not right where you see it visually. When you open your phone, you can see that there's a small corridor before your voice get's into the actual microfon. Anyway, the issue is, that since the protection cover is made of cilicon/plastic kind of material, it just eats up all of the voice/sound and it just doesn't lead up to the real microphone. That's a pretty bad flaw in my oppinion, I can't have a protection back cover to my yellow beauty and it's very sad 
