Nothing confirmed yet. Though based on some shots in recent videos, it looks like it should be at least bluetooth (default) and possibly USB-C on the front.
The new speaker panel has a couple buttons and a volume knob, as well as what appears to be that USB-C port.
there is a lot of reason to believe that every audio component Slate is offering is meant to be Bluetooth connected (either your source, or interconnections).
this makes sense from an ease of offering and universal compatibility perspective.
However, I wouldn't be surprised if some had 3.5" inputs on the rear, simply because the oem had that as original design.
I believe the center speaker has been shown with front USB-C, and it seems likely that could serve as input.
as with most things Slate... we gotta wait... agonizing wait....
I'm imagining the central speaker is connected to power through one of the plugs behind it. The USB-C on the front might facilitate a cord from there to your phone/tablet (to keep your device charged). The side speakers are either hard-wired or otherwise linked to the middle speaker.
All speculation, mixed with wishes, and delivered with luke-warm confidence.