iMac Retina 5K, 27-inch 2017 macOS Big Sur v11.5.2 No output sound device and thus won't play any sound.

Start with the Restart in Safe Mode by immediately holding the SHIFT ket at restart. This will do a repair Disk, clear cache files and only load Apple Software, extensions and fonts. The loading process may take some time and this is normal.

Once logged in - test if the issue is presented.

If Not presented in Safe Mode - this may indicated something in the User Account.

To Further trouble shoot the issue >> Set up users, guests, and groups on Mac for testing purposes.

Log Out of Original account and into the dummy account and test again for the should issue.

If the issue is still present in the dummy account - that pretty much confirms this is a System Wide Issue

Do post back the results of above trouble shooting and where( if at all ) the issue is no longer presented.

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

You are welcome and thank you too.

Do come back to the ASC Forums, ( Apple Support Communities ) in the future, if / or when additional question may arise