They change the music? That doesn't bode well IMO. Also, if they've revoiced it, that means it doesn't have the original Leon (since he's apparently long dead) which can't be a plus.
I got an N64 for christmas and have the original on it, is there any real reason for me to buy the 3DS version?
Yeah, all of the music has been reworked. I originally thought the same that I wouldn't like it, but turns out it was a good update for the most part.
The voicing has a bunch of different actors from the 64-version yes. From what I have found, Fox (also James) and Slippy (also Katt) have their original voices, and a lot of the bosses who speak have the same voices as well. Wolf (also Training Mode Tanuki (who now has a voice)) has the same actor from Brawl. Falco (also Bill and Andrew) has a new voice, as well as Leon, Andross, General Pepper etc.
The VA for Leon is actually still alive as far as I know, he recently did some work for Team Fortress 2 in 2007, so I am only assuming.
It can be seen as disappointing, I had originally wished for everything to remain exactly the same, just with an update the graphics, but honestly I'm not complaining. The voices are well done, and I didn't notice a huge, noticeable difference in the way they sound.
Really, it depends on how much of a fan of the series you are in determining if you should purchase the 3DS version. I am a huge Starfox fan, so I purchased it without hesitation. The graphics alone sold me, because really they are quite amazing in terms of updates from the original. Also gotta factor in the available portability with it being on the handheld. I guess the only real reason to purchase the game would be for the updates, it is a remake after all, same story with people who purchased Ocarina of Time 3D.