Refusing to refund

I bought a sound system on the 25th December 2018. When I got home and tried to play music from the laptop using the audio jacks the speakers gave out an annoying wailing sound. I tried changing the cable but the sound persisted. The tuner receiver couldn't catch any radio station. The following day (26 Dec.) I took the system back and they refused to refund my money. They claim that they can only exchange the sound system with another one. I do not want this sound system anymore so I left it in their shop.