After much searching around, followed up updating multiple drivers to no effect, we discovered how to fix it. The problem is caused by a mouse with a wheel, where the wheel can be used to reduce sound volume but, in this case, gets carried away and does it all the time. The fix is simple – unplug your mouse, reboot the laptop, then when the laptop has re-booted fully, plug your mouse back in again. Hey presto, the volume control no longer slides down!
Hope this helps someone with the same problem – if you happen to come across this post via a search engine and this helps you, it would be great if you were to take a second to leave a comment. It’s always interesting to see who drops by and where they are from!