From what you have stated, it seems that you have correctly connected the TV's 3.5mm audio output to your receiver. Although I would first suggest trying to connect an optical cable from the TV's optical output to your receiver's optical input (if its available), contacting Samsung Service would be your next best course of action.
Back of the TV has a single audio output. I put a Y connector single RCA plug into the TV and the double RCA connectors (Red and White) into my stereo receiver's input. (Same way I had my old Sony connected only it had two output's rather than one.) All I got when I switched to external speakers was a hum. Any suggestions?