ANSWER: The Macintosh startup sound has evolved over the years, but the last three tones are the ones we're playing here. They're from the Macintosh Quadra AV, the PCI-based Power Mac and the iMac (and every model since then, which uses the same tone now).
Let us know (firstname.lastname@example.org) if you have any other suggestions for mystery sounds for future episodes!
Keith Shaw rounds up the best in geek video in his ITworld.tv blog. Follow Keith on Twitter at @shawkeith. For the latest IT news, analysis and how-tos, follow ITworld on Twitter, Facebook, and Google+.