Thanks for the magnificently rapid reply Bruce.
However I don't have a laptop, and my audio sockets are unreachable in my computer cupboard. I need to use the USB ports on the front of the computer. I saw on RC Groups a reference to wires going to the USB ports, and a link showing how this might be done. Unfortunately the link is unavailable.
I don't suppose you know how to wire the lead for the USB ports, or even if this would work?