Hang problem in webControl and SAPI in C#

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C#

Speech-SDK

I've made a browser with WebControl. now I want to read and highlight the text. So I split then sentences and start a loop.now the problem is,with a Synchronous call of SAPI to speak the sentences,every other button of the browser stops working.I can't pause,resume or even exit the software. Every button get hanged. If I use a asynchronous call of SAPI ,this problem doesn't occur,but then the highlighting of text will not work.So , I need to stick with this Synchronous call and also get over with this hang problem. Does anyone have any solution!!!!!!!

for (int i = 0; i < splitSentences.Length; i++)
{
    highlight(splitSentences[i]);
    sound_object.Speak(splitSentences[i]);

}

Please urgent help needed.How can I work out those pause,exit buttons of browser while speaking the sentece....