In Vista I was effectively able to have my own custom startup sound by disabling the (non-configurable) official Vista startup sound and having the logon sound set to what I wanted.
I found I could do this in Windows 7 too - EXCEPT that once I'd set up my auto-login (as I also had set up in Vista), the logon sound didn't play. It would play only if I logged off and then on again.
I rather expect I'm stuck with this situation, but does any nice person out there know of a means to get logon sound to play on system startup even when there is an auto-logon? -- Many thanks!