Add mute fuctions for one sound in OALSimpleAudio

[dyego]

I'm developing a musical book, so didin't find functions to mute one sound using OALSimpleAudio, so I added tree functions to solve it.

  /*
     switch beteen mute and unmute.
     @param filePath The path containing the sound data.
 */
- (void) switchMuteWithEffectName:(NSString*) filePath;

/*
 only mute the effect.
 param filePath The path containing the sound data.
*/
- (void) muteEffectWithName:(NSString*) filePath;

/*
 only unmute the effect.
 param filePath The path containing the sound data.
*/
- (void) unmuteEffectWithName:(NSString*) filePath;