I'm sorry if I confused you more :/ Hmm, try saying it fast and maybe you'll be able to hear the "d" sound. Honestly, I think it just takes more listening and speaking to be able to hear it. Hope this helped you a little^^.
@oj1992 : Ah, I understand where you are coming from. It's actually "nu" but because the way it's said in Korean makes the "n" short, it sounds like a "d." So in English, we would naturally hold the sounds longer like "nnu ggu." But in Korean, they cut the sounds short with their tongue so it has a "thud" sound which makes the "n" sound like a "d." Same goes with 네 (ne) which means yes in Korean. When you hear it, the word sounds like (dae).