On the 310, 400 series, 540 and A5000, there is a connector inside the computer, where you can fit an unfiltered sound output from your computer. This makes high sounds audible, and generally improves quality, but you cannot plug your headphones directly into it. We do not, however, accept any responsibility for the use, or fitting of this device.
Here is the pin connections of this connector:
You should be able to get the components from good electronic supply companies, such as Maplin, or Tandy.