Probably the speakers. To me it sounds alright. However, there is a possibility that because ض is a new sound to you, your brain is trying to associate this sound to the nearest possible substitute. It happens sometimes when people learn a new language.
The sound of ض is distinctive for Arabic in fact. Maybe the closest I can approximate this sound to, is velar D. Try saying "Da" but curve your tongue downward inside to make more room, and when saying it compress the air a bit against the roof of your mouth. Hope that helps!