Note: groups.io will be down for maintenance on Monday, September 26th, starting at 9AM Pacific Time (4PM Monday September 26, 2022 UTC), for approximately one hour.
The way you get OneCore voices is by downloading Language Packs. Settings, Time & Language, Add A Language button. Be certain that the one you're choosing includes text-to-speech as one of its features. I added English for UK, Australia, and Canada. This will also result in a new control showing up in the system tray that allows you to switch keyboards related to the languages you've downloaded.
Now, as an aside, using the dictionary to control this is a cinch for just MP3 or mp3. The regular expression would be [m,M][p,P]3
Though you would, in all probability, need to follow the 3 with the character for whitespace. If you wanted it for MP2 or MP3 then the last bit would be [2,3] or [2-3] as both the enumerated list and range are equivalent in this case. That's the power of the regular expression, and if you wanted it for both then you'd capture that last character so that you could use it in the substitution regular expression.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back