|"There are small output jack sockets on the PC but these do not come from the soundcard".|
Then where do you think they come from? If you have integrated sound on the motherboard the input/output jacks (ports) will be located high in the board itself.
I am not sure what you mean by HDMI sound. HDMI is for video and audio.
Does your receiver have input jacks for HDMI?
Post the computer model or if a custom built computer the motherboard model. If you have actually installed an add in sound card then post that model.
The integrated sound on the motherboard usually uses 3.5MM jacks. You would just use the line out to connect the PC to a receiver.
Depending on what the sound source is it may be stereo, or something more than that.
Radio Shack and stores like it sell many adapters to get from one type jack to another.