检测麦克风可用状态

checkRecordPermission

		if ([[AVAudioSession sharedInstance] respondsToSelector:@selector(requestRecordPermission:)]) {
			[[AVAudioSession sharedInstance] requestRecordPermission:^(BOOL granted) {
				if (!granted) {
				//检测到不可用
					dispatch_async(dispatch_get_main_queue(), ^{
	          //回到主线程做出相应提示
					});
				}
			}];
		}