This was just the thread that I was looking for to vent my frustrations....
Out of all the content that the 3DS version has in terms of reused content, I think the entire sound department (both songs and voice overs) was the biggest offender. Don't get me wrong, all of the new remixes are some of the best in the series (i.e. Rainbow Road, Gerudo Valley, Smiles and Tears, the list goes on), but when I hear all of the rehashes that make up about 50% of the music in this game, it's personally very hard to ignore. I honestly look forward to each new Smash game for all of the new music pieces; if I wanted to play with the songs from the previous Smash games, I would rather go back and just play those games. Especially since every stage in this game only gets two songs, when a straight game rip or prior remix takes the place for a potential new remix, I can't help but feel like the sound team cut way too many corners this time around and decided to rest on their laurels instead.
The only thing that got me more upset than the stage music was that the Brawl victory themes returned (most of which I didn't care for, especially Captain Falcon's), but then exceptions got made for characters like Bowser, Rosalina, Meta Knight and Dark Pit. If the intention was to differentiate those characters with more fitting themes, why couldn't the same have been done for every single character? Almost every character on the roster has some kind of notable theme/jingle associated with them, so I can't think of any reason why such a task couldn't be feasible.
In a similar vain, the strange choices in which characters got new and reused voice overs is not as infuriating as my previous two complaints, but still another prime example of how the sound department got lazy. Like I've said before, the new voices for characters like Pit and Sonic are great, but also make the fact that the rest of the Brawl VOs got copy/paste jobs. The fact that Bowser, Donkey Kong, and Diddy Kong still don't have their official voice actors for two games in a row still makes no sense to me. Even though the characters weren't changed enough to warrant full sets of new clips, a few of them here and there for each character would have been nice, especially since I'll have to hear them for at least the next five years.
(takes a deep breath)
...Okay, long text rant over (for now at least...)
tl;dr: Smash 3DS sounds and music: What's new is great, but there's way too much of the old that the new often gets overshadowed.