ios文字合成语音没有声音

代码：var AVSpeechSynthesizer = plus.ios.importClass("AVSpeechSynthesizer");
var AVSpeechUtterance = plus.ios.importClass("AVSpeechUtterance");
var AVSpeechSynthesisVoice = plus.ios.import("AVSpeechSynthesisVoice");
var sppech = new AVSpeechSynthesizer();
var voice = AVSpeechSynthesisVoice.voiceWithLanguage("zh-CN");
var utterance = AVSpeechUtterance.speechUtteranceWithString("你好你好你好你好你好");
//utterance.plusSetAttribute("rate",64.5);
utterance.setVoice(voice);
sppech.speakUtterance(utterance);
plus.ios.deleteObject(voice);
plus.ios.deleteObject(utterance);
plus.ios.deleteObject(sppech);
运行结果没有声音