speaker - AVSpeechSynthesizer 扬声器开启

Question

连接耳机后，我可以听到通过耳机朗读的文字。但是，如果我断开耳机，它根本不会播放。如何打开此演讲的扬声器？这是我的代码：

[syn stopSpeakingAtBoundary:AVSpeechBoundaryImmediate];
syn = [[AVSpeechSynthesizer alloc] init];
syn.delegate = self;
utt = [AVSpeechUtterance  speechUtteranceWithString:rtext];
float speechSpeed = 0.3;
NSLog(@"Read***** text%@",rtext);
[utt setRate:speechSpeed];
utt.volume = 1;
[syn speakUtterance:utt];