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Update ... sorted it!
I'll just put up the solution in case it helps anyone in the future.
I had checked all the 'normal' mute buttons on the volume sound settings etc and found everything to be switched on, so I thought no problem. I found a page on the web that suggested going to the sound (loudpeaker) icon on the taskbar, right click and select 'open volume mixer'. This will then show the apps that are running and whether they are muted. However, it doesn't show the mute options for non running apps, so I looked at this window whilst Firefox was running and sure enough, the volume was muted. Clicked the mute off and everything works again. I then ran Rail Simulator, minimized it (using the 'window' button on the keyboard) and opened the volume mixer again and there it was muted! Un muted it and everything works again...sorted. Only comment is I have no idea how I muted them in the first place. Must be a hotkey on the keyboard that I don't know about. At least I know how to restore it if it goes wrong again!