Looking good, Gijo. Where are you located, I've only seen that green MDF a time or two in specific applications.
Not sure what you'd be sealing with silicone caulk, but I'd reccommend avoiding it. After all it's not a sealed speaker. Silicone is a finisher's nightmare and assuming you have decent glue joints, adds nothing to strength of cabinet. Even if you're not putting it on finished surfaces, it seems to migrate onto them. Nothing adheres to it well, it causes fisheyes in paint, almost impossible to remove completely, not to mention a host of other problems that aren't easily fixable. If you must caulk, there are better choices.
Can you tell I'm biased? I swear they should sell the stuff only with a prescription and educate people about it's best uses. The labels would lead one to believe it's absolutely wonderful for everything...I would disagree, I've seen as many problems created as solved.
Climbing down from my high horse now...