The following is coming from SEGA’s Stephen Frost in an interview with Sonic Stadium when talking about Sonic Boom:

When asked about the music in the game:

SF: At this time, we are not talking about the soundtrack or the musicians involved in the game. We should have more details at E3, though.

When asked if the 3DS version will feature some sort of connectivity with the Wii U version:

SF: The 3DS game will have some connectivity with the Wii U but details of that have not yet been announced.

When asked about 2D gameplay in Sonic Boom:

SF: As with most Sonic titles, where there is generally a mix of 3-D and 2-D-based gameplay. Sonic Boom will be no different. Given the focus on exploration of this new world for Sonic, we generally wanted to focus on 3-D. However, given our desire to deliver a “familiar but new” experience, you can bet that elements like the 2-D-based gameplay will certainly show up, but maybe with a few twists.

When talking about the multiplayer and whether or not there will be online co-op:

SF: At this time, all we have announced is that the game will support up to two players in the main mode with parts of the overall game supporting up to four players. In minimum, there will always be two characters together, but that doesn’t necessarily mean that Sonic has to be one of them. I also want to confirm that there are no online co-op features currently planned for this game. We felt, given the design of the game, that local multiplayer and co-op would be the more appropriate things to focus on. When people actually get their hands on the game and see how it plays, I think they will more clearly understand why we focused on what we did.