No iMac chime

Please identify the iMac year model and macOS version.

see > Identify your iMac - Apple Support

1) go to Apple menu > select Restart... and see if the iMac chimes this time.

2) if you have external speakers or headphones plugged into the audio port, disconnect and restart.

If those do not help, startup in Safe Mode followed by a normal restart.

see > Use safe mode on your Mac - Apple Support

If it is an older Intel iMac then resetting the NVRAM should reset the sound volume.

see > Reset NVRAM on your Mac - Apple Support

AdArt wrote:

I shut down my iMac while on vacation. Upon return and startup, there's no chime.

If this iMac isn’t an old iMac, here is how to: Turn the Mac startup sound on or off - Apple Support

This section is for iMacs introduced before 2006. Is yours that old?