[jackie_w]

1. Installation of a Win10 Language pack should also result in all Voices in the pack being automatically added as sub-keys below this Win10 registry key:
   

   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech_OneCore\Voices\Tokens

   Use the Windows regedit utility to drill down to the above key and expand it to check the full list of installed Voices on your PC.
   
2. MS Voices available to calibre are stored under a different Windows registry key. The key depends on whether you're using calibre 32-bit or calibre 64-bit:
   • Calibre for Windows 64-bit:
     

     HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech\Voices\Tokens

   • Calibre for Windows 32-bit (including Calibre Portable):
     

     HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\SPEECH\Voices\Tokens

   So, if there are MS Voices in the list from step 1. which do not appear in the list from step 2. we need to find a way of duplicating the registry Voice data to the relevant registry key where your version of calibre will see it.
   ***This is not difficult but it is Windows registry hacking, so proceed at your own risk***
3. Close calibre before you start.
4. Using the en-GB Cortana Voice, named Sarah, as a worked example:
   Use regedit to drill down to the key shown in item 1. Expand this key to see its Voice list and right-click on the missing Voice's sub-key, in this case
   

   MSTTS_V110_enGB_SarahM

5. You will be offered a pop-up sub-menu. Select Export. Then save the file to somewhere you can find it again on your PC. It will have file extension .reg
   Please be aware that Windows .reg files behave like Windows .bat files if you double-click them, i.e. they run, so DON'T DO THAT - at least not yet!
6. Find the saved .reg file and open it in your favourite text editor (I use Notepad++ on Windows)
   See screenshot below. You need to manually edit the 2 yellow highlighted rows containing registry keys
   

   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech_OneCore\Voices\Tokens\MSTTS_V110_enGB_SarahM]
   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech_OneCore\Voices\Tokens\MSTTS_V110_enGB_SarahM\Attributes]

   Depending whether you're using calibre 64-bit or calibre 32-bit make the following manual changes:
   • Calibre for Windows 64-bit:
     Change both occurrences of
     

     \Microsoft\Speech_OneCore\

     to
     

     \Microsoft\Speech\

   • Calibre for Windows 32-bit (including Calibre Portable):
     Change both occurrences of
     

     \Microsoft\Speech_OneCore\

     to
     

     \WOW6432Node\Microsoft\SPEECH\

7. Save the updated .reg file. Run it by double-clicking on it. Windows will present you with a couple of "are you really sure you want to do this" confirmation pop-ups which you'll need to OK.
8. Launch calibre again and open this plugin (or the calibre Viewer). If all has gone to plan, you should now see MS Cortana Voice en-GB Sarah in the Voices drop-down list.

Repeat steps 3-8 for any installed MS Voice currently "missing" from calibre.

Notes:

• Don't be surprised if one day one of your MS automatic Win10 updates "helpfully" removes the calibre-visible registry entries you've just added. In other words, save any edited .reg files you've just created for when you might need them again. Not all MS updates do this but I've had at least one which did!
• If you purchase any 3rd-party Voices their installation file should automatically add the registry keys in the correct place. For example, I own 3 Ivona Voices. All 3 are automatically visible to both calibre 32-bit and calibre 64-bit without any registry hacking being necessary.
• Some purchased 3rd-party Voices may only be available to calibre 32-bit. If you're normally a calibre 64-bit user you can still access them by installing and using a separate copy of Calibre Portable which is a 32-bit application.