Graham,
Ububntu desktop version has the "Gnome ALSA mixer" which can be found under "multimedia" applications. The mixer will list all soundcards on your machine, and for each card there will be a set of sliders to control the different inputs. Unmute the inputs which you want to have in service, and adjust the master volume and individual volume controls to suit.
Typically, your generic soundcard will be listed by brand and model #, and the audio input for streaming audio from the web will be labelled "PCM" and/or "Line".
I use "PureDyne", which is an audio-specific Linux release, so I can only give you general information regarding Ubuntu. An audio enthusiast such as yourself should really be using PureDyne or 64 Studio, because they are released with a real-time kernel and can do some serious audio and video chores.


just looking for clues...
Oren.
http://www.masteringmatters.com