lost sound after update
Lost sound after security update, no internal speaker option for output device sound preferences
iMac 21.5″, macOS 10.13
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Have you done a PRAM reset, CMD+Option+p+r...
http://support.apple.com/kb/HT1379
In fact, do 3 in a row, takes a bit of time.
Intel-based Macs: Resetting the System Management Controller (SMC)...
http://support.apple.com/kb/HT3964
Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?
Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.