If you're taking about the sound files that are the same type as system alerts, I think their extension/file type is .snd (which I'm pretty sure is just a .wav file with a Mac OS extension). You can create/convert these with an application called SoundEdit. These files play within the Finder (no application needed like Quicktime) so they can easily be used with a startup AppleScript, or maybe even just by putting it in the startup items folder?
You can't change the actual boot chime on Mac but you can get these alert sounds to play automatically when the finder loads. Some people in the past built extensions that would play them as the system loads.