Thanks for the advice, unfortunately I have tried all of that to no avail. I'm a little stuck, it's not a great inconvenience to me, but I'm used to controlling the volume via my keyboard and I do tend to view youtube quite a lot. I can't even pinpoint what I may have done.
Yup, I have uninstalled it and reinstalled it. No difference. In fact, now it doesn't even appear in Add/Remove programs when I looked for it to try it again.