The following is a guide to get you up and running with all preferable and usable mobile tts voices, even ones from other dialects of english!
You must be running windows 10 for this to work.
First thing's first, install the language packs required and download the speech voices to use.
Hit windows I, go to time and language, then region and language.
Hit add a language.
The list is a tile view, so go down until you hear estonian and arrow left. If you get into the f's, just arrow left a few times until you hear english, and click it.
Then you'll have to repeat this process a few times to add multiple dialects, but select the following from the list of dialects in your times in this dialog.
Australia, Canada, India, United Kingdom.
Note that on windows 10 version 1803 the add a language panel has been simplified, the dialects show in the list and there is now a search bar. Just type english into the box, go to a dialect and press enter, hit next, uncheck anything but speech, and hit install.
once you've added all those languages, here's another series of steps to repeat.
In your list of installed languages, US or whatever should say default language, while the other ones should say language pack available. If they say searching windows update, wait until they say language pack available.
Click enter on a language. It should say expanded selected.
Tab to the options button.
There may be multiple download buttons depending on which language it is, but tab to the add keyboard button, or if it's not there, the list of keyboards.
Then shift tab and click the download button, that downloads the speech components. You can varify this is the speech components by using your form of object navigation and going directly left of that button.
Note in win 10 v 1803 the download buttons will tell you what they download, so you don't have to worry about obj nav or finding the right button. For india, do not install the language pack.
Repeat that for all the languages. If speech is already downloaded, a settings link will replace that download button.
Now, it's time to install the voices.
You'll have to import these into the registry for things to work, that's where the mvoice.reg file comes in. The reg file has been modified in this latest update to go straight through wow6432node, A.K.A. the 32 bit apps registry, so pressing enter on it should just work. If it doesn't, however, you can still run regedit and import it that way, though with that you'll most likely want to run the following to make sure you land in 64 bit level.
C:\Windows\system32\regedt32.exe
Once your import is complete, you should be able to use all these voices in sAPI5 with any 32 or 64 bit compatible application, and even set them as your default tts voice in normal control panel!
You may wish to varify this with a 32 bit application that can configure the SAPI voice, such as TTSApp, screen readers, or klango, to make sure they import. They will all say microsoft something - english, whatever.
These next steps are to remove all the keyboards those language packs will install.
GO back to the settings app and get to the list of installed languages. That's windows+i, time and language, region and language.
Go to whatever languages don't say default language, this should be anything below the first item, but make sure. They will still say language pack available.
Select them with enter as done before.
Hit remove.
It will disappear, and do that for all the packs you've just downloaded. Don't freak out, the voices will stay.
It has come to my attention that upon a windows version upgrade, at least of major branches like 1703-1709, the hack will disappear. This is most likely because the language packs have been deleted. Just rerun this when that happens, varify that the keys still exist if you wish in HKLM\software\microsoft\speech\voices\tokens. If they do not, make sure you have redownloaded the languages before applying them, otherwise you get a bunch of dead voices.
Note for windows 10 1803: the packs will disappear on restart. You have to keep the languages installed. If you don't want the keyboards in your switch list go to HKCU\keyboard layout\preloads and delete the entries you don't want, I verified the language codes using an msdn article, but all the keyboards will end with 09. You will want to leave the 1 entry alone, and if you've installed other keyboards before doing this they will be in sequencial order. For example, I have the US International keyboard on 2, so I deleted 3 through 7.
Enjoy your new SAPI voices!