I've had more of a play with this now, and it seems that the problem is/was that the "radio" installation set the default settings for my sound card to enable the microphone input
and set a 20dB input boost.
When I finally found the settings for the sound card, I was able to manually check-then-uncheck the 20dB boost box, at which point sound became fine again. This was great, until I rebooted, at which point it got set to the stupid settings again.
So the problem has mutated into:
Does anyone know how to configure the default sound card settings (for my "SoundBlaster Live!") at boot? I
guess it's in the registry somewhere ...