Fix an oversight where sound would never be initialised if a game is started with it off, and then is enabled later.

Before, if a user started a game with sound disabled, the mixer would never be initialised, even if they turn it on later, and they're still in-game. This simply checks to see if mixer isn't null(and that we're in-game via PSP_IsInited) and if it is, it initialises it.

It was just an oversight, really.