Once you have your text file ready, click on the ‘Convert To Audio’ option on the tool bar. You will see the following window:

Select the value for the audio quality from the drop down list. You can also set the values for audio frequency and bit rate. Generally, a higher bit rate will produce better quality.

For more details see this link:
https://www.deskshare.com/help/ts/SpeakingTextToFile.aspx