Spoilers: (Highlight to reveal)
Jax only appears in Konquest mode and you can fight him a couple times. He looks pretty much like he did in DA. Sorry :( unless he's a SU he isn't playable.
YES! Lui Kang is playable and he friggin kicks ass! His main appearance is a zombie, i.e. rotten flesh and such. His alternate costume is just his normal MK appearance. In a way, there are two of him, in Konquest mode you can train with his Ghost, but from the looks of things it's just him but a see through green color. It's rumored that Ghost is a SU or a hidden costume for Liu.
The Krypt... is once again very dissapointing. It's pretty much just a smaller version of the DA Krypt, which is a good thing because it means there's a lot less junk. Then there's the Krypt Keys that you find in Konquest mode, you just press R inside the Krypt once you have them and it automatically takes you to the koffin it unlocks. So other then all that, and that you can see all kinds of characters running around the koffins it's nothing special.
The music, I think, is one of the great aspects of the game. They really put a lot of effort into matching the music to the levels and they did a bang up job. Nothing like a good song to inspire some Grade A ass whoopin!
The sound effects are pretty good and match up with the levels/deathtraps/fatalaties/hara-kiris pretty well. Still not perfect in my opinion, but better then before. As for vocals you'll hear a LOT of different voices in Konquest and I think they did a great job matching them to the characters. Outside of Konquest you really just get a whole lot of Shujinko as he's the narrarator of the Endings (Sorry, no FMV endings this time either). I personally am very satisfied with the vocals in this game but I've heard from a lot of people that hate them so I guess it's just a matter of opinion.
As for other things I have to bring up the additional modes, i.e. Chess Kombat and Puzzle Kombat. These are mainly just a matter of taste depending on whether or not you like that kind of game. Both games are pretty good and they provide you some nice variety from Konquest and Arcade and let you have lots of fun, especially when going against your friends. But like I said, it all depends on you whether you enjoy them or not. I, personally, don't like Puzzle Kombat while my girlfriend is madly in love with it. I've NEVER been able to beat her at Puzzle and likewise, she's never been able to beat me at Chess. It's because of these modes that this game is DEFINITELY buyable because you'll always have something else to do instead of just fighting.