You can add a new startup sound pretty much like changing the startup screen, but it's not likely your friends did this if they weren't ResEdit type hackers.

Click to expand...

Yeah, you can put a System 7 Sound format file in the Startup Items folder and have it play after the Mac boots up, or I think earlier versions (around System 3) even supported a custom "StartupSound" file that would play while the machine started up. But you cannot replace the boot chime, its burnt into ROM.