Apple - Can the macOS startup sound/chime/ bong be changed on a MacBook Pro?

You technically can't...

But there are a few things you can do instead.

Using a few techniques I picked up on the interwebs, I've figured out a way to both

1. Disable the boot chime (seriously, I don't recommend this. This is here for debug purposes, disabling it can be a very bad plan)
2. Run a sound at login (not boot - for some reason, at least in El Capitan, using Afplay at boot through crontab and LaunchDaemons is not working. If someone else can get it to work, lemme know, I'll add it)

Disabling the boot chime

^{I seriously recommend not doing this. Debug is important, and if there's a hardware issue, you need to know what startup chime you have. Turn back, ye who fear.}

1. Go into Terminal.app.
2. Run the command

   sudo nvram SystemAudioVolume=%80

   • This can be undone with

     sudo nvram -d SystemAudioVolume

   • Note that this does not work in OS X El Capitan (because changes ಠ_ಠ)

The boot chime is now disabled.

Adding a login chime

1. Go into Automator.app.
2. Hit New Document
3. Select Application, and hit Choose.
4. Drag Run Shell Script in the second left-hand column into the main pane.
5. Type in the command

   Afplay -q 1 /path/to/sound/file

   obviously replacing /path/to/sound/file with the path to the sound file.
6. Save as an Application and place it in your Applications folder.
7. Quit Automator.app
8. Go to System Preferences.
9. Click on Users and Groups.
10. Click on your user in the left-hand-most column.
    • If necessary, click the lock in the lower-left corner to allow for a preferences change.
11. Click the + underneath the main pane on the right.
12. Select the Application you just made and hit Add.
13. Click the Hide checkbox.

That's it.

You now have no startup chime^{(use caution)} and a login chime. Hooray!