oh nice find t-bone

If I had to guess, the speaker disables controls for all players to some extent. Maybe just makes the steering wonky. Maybe makes everyone decelerate. Hard to say.

The 8 seems likely to be the same thing as the 7 from MK7, which brought out seven items. Only now it'll include the speaker....so eight items.

maybe it shoots a wave to loose gravity????

or shot a wave that let you jump over an opponent?

You can actually see Bowser holding the speaker box item in the new trailer. He's carries it while driving behind Yoshi as he pulls out the Piranha Plant. Unfortunately the screen cuts away before we see what Bowser does with it.

My guess for the speaker would be so that Wario could ride through my neighborhood playing the song "Back Dat Ass Up" early in the morning and waking me up from a good night's sleep.

It'll probably work like that quake weapon in the Wipe Out series, though.

Yeah probably.

Though, wasn't that 'Lucky 7' item there in Mario Kart 7 because the number '7' was important in Japanese culture? Or maybe I just dreamt such an explanation existed sometime ...

Hmm, that's a pretty good thought actually. Never even considered that!

I'd still imagine 8 will be more of the same, though. I was never a big fan of it myself but I guess some were. 

It's likely, yes. Personally I didn't mind the '7' much, was useful most of the time you got it, but I could've done without. It kind of felt like a filler, as if they couldn't think of anything else.

Yeah, about what I thought. There's gotta be a better choice than the 7. I liked the Super Mushroom...