Some sounbars have to directly plugged in, but I think some are wireless.
I can thoroughly recommend getting a soundbar. You are right, flat screen TVs often have speakers on the back. A soundbar throws the sound forward, and often come with a single woofer speaker for added bass.
....however, some inexpensive soundbars can be no improvement at all. When Which? last tested soundbars, there were some that they rated "Don't Buy" because their sound quality was very poor.
beware they will fob you off with the most expensive one and when you tell them your tv is old ... watch out you'll get the hard sell for a new £1000 job !