Sound don't work if disabled before starting a game

[TheDax]

It's a bug an oversight. The sound mixer isn't being inited at all if sound isn't enabled. It can be fixed in two ways that I can think of, though I'm not sure which is best yet:

1. We can init the sound mixer after enabling sound(and checking to see if Mixer is NULL, and if the emulator is actually running a game via PSP_IsInited to avoid a double init).
2. Init the sound mixer regardless of if sound is enabled or not and simply set the volume to zero so no sounds can be heard, then change the volumes if sound is later re-enabled.

The second option would override existing user settings on audio volume, though. I think I'll go with option 1.

Edit: https://github.com/hrydgard/ppsspp/pull/3165