如何使韵律持续时间适用于所有语音性别？

如何解决如何使韵律持续时间适用于所有语音性别？

我正在将 SSML 发送到语音合成器，但如果性别是男性，它只会尊重韵律持续时间。这是一个代码片段：

using System.Speech.Synthesis;

SpeechSynthesizer synthesizer = new SpeechSynthesizer();

string slow_ssml_male = "<voice gender='male'><prosody duration='8000ms'>american one twenty three cleared to land runway one left</prosody></voice>";
string slow_ssml_female = "<voice gender='female'><prosody duration='8000ms'>american one twenty three cleared to land runway one left</prosody></voice>";

PromptBuilder builder = new PromptBuilder();
builder.AppendSsmlMarkup(slow_ssml_male);
builder.AppendSsmlMarkup(slow_ssml_female);
synthesizer.Speak(builder);

我错过了什么？