My theory, and the most untraceable way to do it today, would be to use recordings out of movies or video games, there must be dozen of them available.
Unless you have those movies and video games already on your hard drive, downloading them will create a log somewhere that could be traceable. :/ Windows TTS on a device not connected to the internet is a better choice. (I don't know why we are trying to give them ideas!)
No need for the games you have all the material on youtube.
That's my point. If you go search on YT for those videos, you're leaving a digital trail that can be easily traced back to you.
Simply using the "say" command on a mac would work.