"Choi" isn't pronounced the way it looks, so don't be deceived! xD
It would be more like "chwae" (like an "eh" sound). But when people say it fast you would hear it more as "chae" without the "w" sound.
It really is not "choy," but don't worry cuz I was confused at first too, but I got corrected soo many times by my Korean tutor that "choy" is actually the wrong way to pronounce it.
For "seung," the "eu" sound is a bit hard for me to explain, but I'll try! Sorry if the explanation's kinda weird lol. So, you know when you say "uh" your mouth should be slightly open, well you would have to close it a little more. Well just try it when you kinda smile with your mouth slightly open.. Ok that was a really bad explanation -__-; so..
Here's a video and at 0:20 and 0:43 is the "eu" 으 sound you should be looking for ^^;
http://www.youtube.com/watch?v=3OkBHAbQLns
And then you just put all the sounds together.
For "hyun" it's pronounced the way it sounds, the "u" should not be like an "ooh" sound but like an "uh" sound.
In Hangeul T.O.P.'s name is 최승현
^^