No sound from speakers

I'm in process of adapting a refurbished desktop HP pc to play digital files from my old cd collection. All going well well- except that the sound is coming only from the built in speaker! I plugged a wye connection cable with 3.5 mm plug into the light green audio jack on the pc (it was marked with a tunes icon) and at the other end of the cable (the two RCA plugs into the aux. stereo jacks on my receiver). First problem: this plug-in did not silence the internal speakers in the pc; 2nd problem: no sound comes from the room speakers. I don't find any switch on the pc that would kill the internal speakers, but I can probably work around that. The one that has me stumped is the failure to get any sound from the room speakers; I've tried cranking up the output on everything available - no help; tried the audio plugs in the phono, CD, tape input jacks - no change. Rechecked the tightness of all plug/jack connections, and determined that the room speakers and the audio receiver/amplifier are working (from the radio function and phono output)

Thinking about trying a different pc to help isolate the problem. Any ideas and/or insights will be greatly appreciated