Some time ago, before practically all cabinets came with front audio and USB ports, there used to be an add-on card that would solve your problem. I don't know how easily it's available these days.
I'm not talking about a PCI card. It's meant to be fitted on the front of a cabinet and usually has two USB ports as well as audio input and output sockets. It carries cables with connectors for the motherboard USB and front audio headers.
This link shows such a product. It's at eBay US, so it's meant only to give you an idea:
HP USB & Audio Front Panel with Cable 337242-001 - eBay (item 390255559603 end time Oct-30-10 22:11:51 PDT)
This one is intended for a branded computer, so the connectors are in blocks. Generic ones usually have separate connectors for each wire (like the front switch and LED connectors), marked with something like L-OUT, R-OUT, GND, etc. You can match each pin with the help of your motherboard manual.
The problem with such products was that there's no physical standard for the way such devices are fitted on the cabinet front, and some improvisation is usually necessary to fit them.
Ask around in your local computer shops. Some shop may still have such a product. It should cost just 50-100 bucks. Alternatively, you may be able to buy an old cabinet cheaply and take the front connectors from that.
If you find the front attachment and need help with the pin connections, post back. However, I don't come to TDF regularly these days, so you can also mail me at pjdd(at)rediffmail(dot)com