Ian,
I'm not using the basic editor PD Express - I'm using PD6.
open PD
Place a video/audio in the timeline - Aquarium.mpg is a good starter
Click on the media file (just to highlight the file)
You should have an audio icon button to the left of the Media Library - click it
A selection of slider controls should be available - move them up to increase.
The express version can only raise or lower the audio - nothing else.
PD6 has more control over the video etc and is a better video editing tool.
Dafydd