Very rarely can little bits an pieces of Al's true singing voice, without all the whimsy and decoration, be heard in his songs. Most often can be found in live situations or where Al is *trying* to *sound* relatively serious, like in his anti-love songs.
Very close examples found on: Al-TV (Bad Hair Day) live performace of Since You've Been Gone, CNN's Showbiz Today live performance of It's All About The Pentiums, You Don't Love Me Anymore
Tenor (True)
Judging from all I've heard (so far) of Al's singing talents, Al would be classified as a 1st Tenor- the name given to the highest male vocal range; equal to the lowest female vocal range.
Abuses:She Never Told Me She Was A Mime, Buckingham Blues (big time!)
Borderline:christmas At Ground Zero, Slime creatures From Outer Space
Examples found on: Headline NewsFalsetto (Baritone)
Same concept, only Al is going lower than his 'normal' range.
Examples found on: Peter And The WolfReal Accents
Like with impressions, Al will sometimes also go for the accent, with or without the impersonation.