Fast Company has an interesting presentation on the importance of audio in branding. It makes sense, especially given that people take in information in different ways - some will more easily associate a sound with a brand, some with a logo and some with a tag line.
I'm not sure about including McDonald's here - "I'm lovin' it" is relatively new (when you're talking about brands that have been around for decades), and personally, I associate it more as a tagline than a jingle (and Justin Timberlake?!). It seems about as timeless as "Whassup?!"
Intel and, for moviegoers, THX, are by far the most recognizable.
AOL...Can't dispute that "You've Got Mail" is iconic, but it's become synonymous with email in general, not just AOL. Besides which, the ability to customize mail delivery sounds (or ban them entirely) renders this one irrelevant. Seriously, do you know anybody who still gets that generic message? Even back in the mid-90s, we were recording celebrity voices to cover for the generic computer voice. If anything, it's now either ironic or retro, kinda like "I can't believe I ate the whoooooole thing." I'd say AOL's start-up sound is more significant, though I can't bring it to mind.
Windows - though I haven't tried Vista, being a Mac gal and all - has a very distinct audio brand.
NBC - definitely the most recognizable audio signature in television.
CNN - James Earl Jones's participation led to a whole spate of JEJ voiceovers (and sound-alike voiceovers), because his voice is so resonant with authority. I dunno, though. The "This is CNN" pronouncement seems so... 1990s to me. Maybe because they always have all those flashy banners declaring the Crisis! Du! Jour!
Mac startup is pretty distinctive, though non-Mac users wouldn't know it.
Where does a tag line or brand word leave off and a brand "sound" pick up? Yahoo! has the best combination of the two (annoying as it is).
Honestly, I think the best "brand sound" around is that double bass line in the intro to any of the Law & Order shows, the one they play between scenes. In 1995, I used that as my "critical stop" sound (back in the Windows days), until it gave me so much anxiety I had to change it.