I built this into a little cookie tin and added a volume knob and a pilot light, but these are optional.
It helps to have a little bit of soldering experience. The amplifier board is already built, but soldering wires onto the headset PCB can be tricky if it's really tiny. The one I used wasn't horribly small, but newer ones may be harder to work on.
Step 1: The USB Headset Dongle
To find the right connections, you have two options:
1) Cut the cable to the headphones and find the right and left audio output wires. An oscilloscope helps here.
2) Carefully crack open the dongle and find the connections on the circuit board. If your lucky, the pads will be labeled. My board said "R, L, and G", but I checked them anyway.
You want to make sure you don't have the microphone line because that's an input and not an output.
You may have to trace out the board with an oscilloscope to find the outputs. Also, for this step I like to send out a sine wave because it's easy to see on an o'scope. What I did is plug the USB end into a PC and run Audacity (a free audio editing program), which lets you output sinewaves. Audacity has Windows, Mac, and Linux versions.
In my headset, the wires to the headphone were too fine to solder, so I removed them and used a different cable. You'll need a 2 conductor wire with a shield if you replace the audio signal wires.
Step 2: Amplifier Board
These amps have a loyal internet following, and if you want to go further and add some "golden ear" modifications, check out the DIY Audio Forum: http://www.diyaudio.com/index.php
You will also need a 12 Volt, 2 Amp power supply to power the board. I used a surplus wall adapter, but any clean 12V 2A supply will work.
Step 3: Diagram
Step 4: Wrap Up
1) The speaker output "-" termnials are NOT the same a GND. They can't be connected to GND nor to each other. That means this amplifier board cannot be used for headphones, which usually have a common ground for right and left speakers. if you buy the same board I used, it will come with warnings about this.
2) I had some noise and had to change the plastic box shown holding the dongle to a metal one. I also connected this case to GND on the dongle PCB. If starting over, it would probably be best to put the dongle inside the amplifier box. That would also make a neater set up, but would require a longer USB cable and bit more soldering.
3) Sound quality - I'm not a "golden ear", but I know Hi-Fi pretty well and am aware of the typical issues that ruin sound. From what I can hear, the DACs in these headsets are pretty good, even in the cheap ones. The reason the headsets sound poor (Hi-Fi wise) is more due to the transducers than the electronics. When you grab the signal and give it to a decent amp and speakers, you far exceed what computer speakers can do.
4) On any reasonably late model Mac or Windows (I don't know Linux, but probably that too) you can plug in the USB plug from any headset and it should be instantly recognized as a USB audio device. You don't have to install any drivers or other software. You may however have to go into your computer audio or "Sound" settings (in "System Preferences" on Mac, or "Control Panel" on Windows) and select the "USB audio" output (or similarly named) device from a menu. On my no-name headset, it showed up as "Unknown USB Audio Device".
Have fun and don't throw out those busted USB headsets!!!