Garbarled intern microphone on 2011 IMAC

Please navigate to the Mac App Store or navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

You have a couple of issues.

1 - you've installed McAfee software which is totally unnecessary and tends to severely compromise system and application performance. Uninstall it according to the developer's instructions. When done uninstalling rerun Etrecheck and look for any macafee files left over. Should not be any.

2 - you have the minimum RAM in your Mac, 4GB. That will seriously affect performance.

After uninstalling MacAfee, remove all items from your User Login Items preference pane. Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down.  Reboot normally.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

Now try FaceTime and check to see if the audio improves. If it does then you can add items back to the User Login Items one at a time and test FaceTime again.

Read this user tip for how to protect your Mac: Effective defenses against malware and other threats - Apple Community   

Try downloading and reapplying the macOS High Sierra 10.13.6 Combo Update and see if that will help.

That should be your account user name and password. The one you setup the first time you used the Mac.

After you run Etrecheck copy the report

and paste into your reply using the Additional Text button:

OT is 100% correct, please follow his advice!