I have my sound card's headphone output connected to the left & right AUX inputs of my Sansui AU-505 audio amplifier. It works fine. Don't use the magnetic pickup (vinyl disk turntable) inputs. I set the PC (Windows) volume control about midway. Then I adjust the volume on the amplifier just like I was listening to any other audio source like a CD player, DVD player, cassette deck, FM tuner, etc. You would not be in any danger of destroying the computer or amplifier or speakers. I do not see why you would want to connect back to the computer though. Perhaps you can explain more about what you are trying to do.
Reading between the lines, are you trying to use an electric guitar or a microphone?
