Fix for SimpleAudioEngine::getEffectsVolume() issue on linux

SimpleAudioEngine::getEffectsVolume() is returning the background music volume when run on linux. The values on Android seems correct.

Here is a test case for the issue:
````
float newVol = 0.99199f;
CocosDenshion::SimpleAudioEngine::sharedEngine()->setEffectsVolume(newVol);
float sfxVol = CocosDenshion::SimpleAudioEngine::sharedEngine()->getBackgroundMusicVolume();
CCLOG("sfxVol: %f", sfxVol);
CCLOG("newVol: %f", newVol);
CCAssert(sfxVol == newVol, "Effects volume should be the same as the new volume level.");
````

I'm running on Kubuntu 12.10, using HEAD of the master branch.