Can speakers go into a headphone socket?

They were not designed to be used that way so don't do it! Speakers have a much lower impedance than headphones so your MP3 player, DVD player (or whatever device it is that just has headphone sockets) may be damaged if you plug speakers into it. That's because the speakers will try to take much more power from the headphone socket than it may be capable of giving. The correct way to drive a pair of speakers is to use a booster amplifier which has been designed to plug into the headphone sockets and has its own separate source of power from batteries or a mains adaptor. Or just use one of the many "booster boxes" which come complete with an amplifier, neat mini-speakers and sometimes a mains power supply too.