Can I change the startup sound on macOS Big Sur?
On an Intel Mac, the startup chime is part of the Mac's firmware. You can't just replace it, like you could a normal audio file.
Think about this logically—the startup chime plays as soon as you turn on your computer, before it loads the operating system. And if you wipe your hard drive, the sound will still play. How would that be possible, if the chime was stored on your drive?