I already think the power-on noise sounds like an obnoxious fart. I can never remember how to suppress it so I refuse to turn a macbook on around other people.

I may as well install this to reaffirm that yes, this is an Apple product (in case you fail to see the prominent logo) and yes, I am better than everyone else for owning one (/sarc).

Open System Settings, click "Sound" in the sidebar. Under the "Sound Effects" section, you'll find a toggle labeled "Play sound on startup". Turn this off to permanently disable it. Otherwise, the startup sound is tied to the audio level you had before last shutdown.