"It don't..." is bad grammar, but works in a pop song (try singing it with "doesn't"... it sounds strange!) This is a whole can of worms, but basically pop songs aren't usually good sources of English. Some writers break rules just to look "cool".

Bruno Mars is saying the song no longer makes him happy because it reminds him of the girl he is singing about. It doesn't sound the same because it sounds sad to him now. The reason he says the bed feels bigger is also because she is no longer sharing it with him. These verses are about Bruno feeling lonely after a relationship ended.