The X-ATC-Chatter user guide has an extensive section on how to get more voices. It will tell you how to install a number of free Microsoft voices. You can also purchase SAPI-5 voices from Ivona and Cereproc:
https://nextup.com/ivona/index.html
https://www.cereproc.com/en/buy-voices
Amazon also has an Amazon Polly Windows Plugin that will install the Polly Text to Speech voices as SAPI voices. This is a subscription service based on the number of characters that are converted but there is a free tier available with limits. The Polly voices are very impressive and it may be worth it to give them a try as an alternative purchase option. There were one or two voices that seemed to generate errors but you can simply unselect them for use on the Simple ATC Voices tab if they don’t generate speech with the Try button.
Provisioning Amazon Polly is a multi-step process so read the instructions carefully. Also, make sure you are comfortable with how the free tier works and what you will be paying should you exceed the free tier limits.
https://docs.aws.amazon.com/polly/latest/dg/sapi-plugin.html
