Dell Inspiron 1545 :: F12 Would Skip To Next Track In Playlist

I have a Dell Inspiron 1545 laptop. On the keyboard the F1-F12 keys double as multimedia keys - so if you press just F5 for example, you'll raise the screen brightness, but hold Fn and press F5 and it'll be a normal F5 press (e.g. will refresh a webpage).

The Multimedia functions of F10, F11 and F12 are Back, Play/Pause and Forward respectively. These have worked for years, for example if I had a song playing in Windows Media Player and hit F11, it would pause the song. F12 would skip to the next track in the playlist. Recently I must have unknowingly changed a setting somewhere because this no longer seems to work. Bizarrely, every other Multimedia key works, so F5 still raises the screen brightness for example.