"Hello to everyone! I've just bought a new Etherwave standard (I am a new player so please forgive me not understanding any difficult terminology), however I am faced with a problem, so I would appreciate if you could help me!! First of all, I connected the theremin to my headphones and I hear a constant sound. The volume antenna seems to be fine, however the pitch antenna does not correspond to my hand movements at all (I did the same with a sound system and I had the same result). I don't know what to do. To exchange the product or it can be fixed easily? Thank you in advance!!" - LeGrandNoir
That doesn't sound healthy and I can't think of anything that you could be doing wrong that would cause this. If you bought this new and you are in the US try giving Moog a call to see if they can offer any advice. If you are getting a constant pitch that doesn't respond to anything that you do with the antenna, including touching it, then something is probably open/shorted in the variable oscillator or its antenna inductors. Even if you have to return it to the dealer it's a good idea to have talked with Moog first, if only to avoid the "you're doing something wrong" conversation that often comes when requesting the return.
If you bought this second-hand (I know you said it was new) and you are on your own then you may want to take a more diagnostic approach. Either way, it probably wouldn't hurt to take a peek inside to make sure that it isn't just the antenna lead disconnected from the pitch-side plumbing fitting. If the constant pitch that you are getting is extremely high and is at its lowest when the pitch knob is fully clockwise, there's a chance that this could be it. But that's a pretty sturdy connection, so maybe this is unlikely.
In any event it probably doesn't hurt to look inside if you are interested. Moog supplies a tuning tool (don't use it yet!) and pretty much expects/assumes/encourages you to get in there sooner or later, so I don't think that pulling the cover on a brand new but non-working theremin to look for obvious shipping damage is going to void anything. But still, talk to or email Moog if you can.