Computers traditionally have a boot sound that plays as soon as they are turned on. If this does not apply to your Chromebook, you can enable it manually from the settings.
On your Chromebook, click the status section in the lower-right corner to reveal the Quick Settings panel. Select the gear icon to enter the Chromebook Settings menu.
Scroll down until you find the "Advanced" button and switch to reveal more options.
Locate the "Accessibility" section, and at the bottom, go to the "Manage Accessibility Features" section.
At the bottom of this page, find and enable the "Play Sound on Startup" option.
If you have problems accessing this expensive menu, you can search for the name of the option from search bar at the top of the Settings menu.
Are you ready. Your Chromebook will now play a short boot sound on the login screen each time you boot. Note, however, that the sound will not "knock" when you just wake it up from sleep.
The volume of course depends on the volume levels of your Chromebook. Therefore, if you want to turn it off or on you are doing less annoying, all you need to do is turn down the volume on your Chromebook speaker before turn off the computer.
Once installed, click on icon from the extension tray in the upper right corner of Google Chrome.
Select a startup sound from the drop-down list, then click "Save".
Source of information: howtogeek.com