I hope they integrate the soundtrack from the previous games...

[Ryu Myuutsu]

Brawl's soundtrack by it's own was already outstanding and on top of that it integrated most of Melee's music as well. I hope that at least for the Wii U version they do the same and have SSB4 absorb Brawl and Melee's soundtrack, it has already happened with some of the previous music.
Smash Bros. tends to have one of the biggest video game music collection in all history and it would make me sad if most of the previous tracks wouldn't make it.

 

[NintendoKnight]

Friday's pic of the day stated this:

"Pic of the day. The 3DS version has an excellent Sounds collection! The design looks nice for a sound test screen, doesn't it? The font color for the "Remix" text on remixed songs comes in pink or white. The pink ones are songs arranged for this version of Super Smash Bros., while the white ones are remixes made in earlier Smash Bros. games. This means there are reoccuring songs from past releases."

Yes, it's been confirmed that past songs will return.

 

[Wanderer from Roseyard]

I hope they hang onto every arrangement they've done for Melee and Brawl, though I could stand to see some of Brawl's "imports" go, or get remixed. I want that OST to gather across sequels like a Katamari ball.

 

[Raijinken]

I'd expect to keep most of the major songs across games (and they'd better hold on to Fire Emblem Melee for all eternity). That said, as much as I liked Shin Onigashima in Brawl, I just can't see them carrying it over each time.

 

[Mechonis]

As cool as it would be for them to import ALL of the songs from the previous games, if they need that memory space for something else I'd rather that they don't do that personally.