Marty O'donnel works for Bungie, not 343, why did anyone assume he would be back? The few videos that have been released feature some obviously new Halo music, and it sounds fine to me.
Most of the core guys at 343 came from Bungie; I'm sure they'll do a good job.
As for going CoD style with multieplayer, I'm still okay with it. For one thing, having a perpetual lobby will be a huge improvement. Nothing sucks more than having your entire team drop out in matchmaking. And the instant respawn times will make the games faster. But it will still be Halo: Spartans with shields and armor abilities.