New answers tagged troubleshooting
You should see if Apple Diagnostics finds anything wrong with your RAM. Turn off your computer, disconnect any external peripherals you have, press the power button to turn your computer on, and immediately hold down D until Apple Diagnostics begins.
In order to try to localise the problem, try adding an additional user to your iMac, and see if this new user also has no sound. If the new user has sound, then you can be sure it's not a hardware problem. If the new user has no sound, then it looks like it's a hardware problem.
I went to the free memory section, and saw that I had only 400MB free (of 16G). So I deleted a few apps, pics, videos etc, and then I could easily send email. Again, not sure that this will work for everybody. If your email settings were right, and everything was working right (you did not change any passwords or servers etc.), this fix may work for you.
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