Hi, Malcom.
Does SMPlayer listen for media keys? That is, if you have Play/Pause/Next/Prev Track buttons on your keyboard, does it react to them? Most media players have this capability and do not require the app to be focused and active to be addressed.
Windows applications must be in focus and active to receive input (keyboard and mouse). That is, unless they are actively listening for specific keyboard events (hooking the keyboard... like hotkeys in VoiceAttack) - again, most media players do this out of the box by, 'listening' for media keys. VA can certainly simulate media keys, even if you don't have them on your keyboard.
On a side note, some applications can address specific out of focus apps with some level of, 'hack' (like multiboxing software), but that is not the normal nature of Windows apps, so, VA is not set up to do that.