录音错误 51: CreateDataFile failed 930: Couldn't create a new audio file object

产生原因：

创建录音文件的时候需要先在doc目录里面创建存放录音的文件夹，不能直接在doc目录里面创建录音文件。

错误示例：

NSString *path = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];

path = [path stringByAppendingString:@"myrecord.caf"];

NSURL *url = [NSURL fileURLWithPath:path];

正确示例：

NSString *path = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];

path = [path stringByAppendingString:@"/custom/myrecord.caf"];

NSURL *url = [NSURL fileURLWithPath:path];