I'll throw in my own two cents with the revealed stages so far, but I won't prove links, because, well, most of them are around here already ;p
Mario Circuit
The Mario Circuit theme from Super Mario Kart, maybe with the Super Mario Kart title screen thrown in. Alternate would definitely be that over used song from Double Dash.
Daien Castle
Not that familiar with Fire Emblem, but I will bet dollars to donuts it's some form of a battle theme/recruiting theme. Probably even "Together We Ride!" like Melee.
Sky Palace
For 7 long years I've waited to hear Kid Icarus done up in a Smash game... And, ever since, I've had it planned out ;p The Underworld theme remixed with the Main Title. An alternate could be the flat out Sky Palace theme on its lonesome.
Halberd
Has to be a mixture of Halfmoon/Mecheye/Escape from the Halberd. I'd love to hear Meta Knight's SWEET theme song from the credits too, though.
Shadow Moses Facility
Duh, the MGS theme ;p
Yoshi's Island
Another stage idea I've been waiting to see for a while. Would have to have to HAVE TO have the Athletic music from Mario World 2, remixed with the Title Theme possibly.
Pokemon Stadium
Same as usual, only... better
Cornerian Orbit
Same as usual, only... better
As for potential stages...
Ancient Edo
Nazo no Murasamejo Theme, done crazy Japanese folk style with a shamisen, taiko drums, gong, crazy guys yelling... everything
Boxing Arena
Punch-Out Training Theme mixed with the Fight theme.
Mt. Dedede
There's no excuse to NOT have this be a stage, and there's also no excuse to NOT have Dedede's Fight music!
Bowser's Castle
Mario 64 Bowser Battle music as a main, but two alternates: one is the Doomship theme from Mario 3 remixed with the Mario 3 Fortresses; the other is the Mario 1 Classic Castle music (NES style)
Wario... something. Ware Inc.? Ware Factory? Palace? Castle? Kitchen Island? Diamond City??
Depends. I'm still praying that Wario keeps his Wario Land moves intact, and would love to see the Wario Land theme pop up in Smash in some form. But the Wario Ware music ain't bad either, so we'll see.