如果使用的AVAssetExportSession只支持合成m4a文件，其他的各種庫也都是從caf、pcm合成的

所以這里使用數據流的形式把MP3文件轉成NSData，然后再進行拼合。

//音頻文件路徑

NSString *mp3Path1 = [[NSBundle mainBundle] pathForResource:@"mouse" ofType:@"mp3"];

NSString *mp3Path2 = [[NSBundle mainBundle] pathForResource:@"large" ofType:@"mp3"];

NSString *mp3Path3 = [[NSBundle mainBundle] pathForResource:@"strong" ofType:@"mp3"];

//音頻數據

NSData *sound1Data = [[NSData alloc] initWithContentsOfFile: mp3Path1];

NSData *sound2Data = [[NSData alloc] initWithContentsOfFile: mp3Path2];

NSData *sound3Data = [[NSData alloc] initWithContentsOfFile: mp3Path3];

//合并音頻

NSMutableData *sounds = [NSMutableData alloc];

[sounds appendData:sound1Data];

[sounds appendData:sound2Data];

[sounds appendData:sound3Data];

//保存音頻

NSLog(@"data length:%d", [sounds length]);

[sounds writeToFile:[self filePathWithName:@"tmp.mp3"] atomically:YES];