Looking through Marty's diagram for the audio wiring, i noticed that there's a road speed signal wire. Is the function of this to keep the volume more or less consistant whilst compensating for cabin noise at speed?

If so, mine might be dodgy. I find that my subwoofer springs into life more when i get to 50mph although the rest of the system volume doesn't change. Basically just makes the sub louder but then cuts off again.

The DSP does have some noise related stuff. All the RTFM says is:

_Automatic features
’Speed dependent volume and sound
equalisation’- The volume and sound
equalisation levels will automatically adjust to
compensate for road noise.

’Speed & volume dependent compression’-The
audio system takes into account road
speed and any quiet audio passages, to
improve volume and sound quality.

’Dynamic headroom management system’-The
audio system monitors both the input
and the output for the amplifier, thereby
optimising its performance to ensure
maximum output with minimal background
’noise’.
_

would you suggest that snipping the wire would put me in charge of my own audio then?

Well i say snip. I suppose temporarily removing the yellow/red wire and see what happens would be the first port of call