Quick guide

1. Open speech settings
   (Or press Win+R > ms-settings:speech)
2. Add voices
3. Wait until the voice is downloaded and restart Soundpad.

Detailed guide

Open Windows preferences from the start menu and select Settings.

Select Time & Language.

Select Speech from the menu on the left.

Scroll down and click Add voices.

Find the voice you want and click Add.

Here's a sample for the Japanese voice.

Scroll down and wait for it do get downloaded.

The easy part

1. Download and install Microsoft Speech Platform.
2. Download and install additional voices. Download only files with TTS in their name, e.g. MSSpeech_TTS_ca-ES_Herena.msi.

The difficult part

1. Export the Tokens registry key by running this command from command line:
   reg export "HKLM\SOFTWARE\Microsoft\Speech Server\v11.0\Voices\Tokens" "%TEMP%\tts.reg"
2. Open an explorer window and navigate to the %TEMP% directory.
3. Edit the exported file tts.reg by right-clicking it > Edit.
4. Search for all occurrences of Speech Server\v11.0 and replace them with Speech.
   Here is an example how the file looks before and after the changes:
   Before

   Windows Registry Editor Version 5.00
   
   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech Server\v11.0\Voices\Tokens]
   
   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech Server\v11.0\Voices\Tokens\TTS_MS_ru-RU_Elena_11.0]
   @="Microsoft Server Speech Text to Speech Voice (ru-RU, Elena)"
   "419"="Microsoft Server Speech Text to Speech Voice (ru-RU, Elena)"
   "CLSID"="{a12bdfa1-c3a1-48ea-8e3f-27945e16cf7e}"
   "LangDataPath"="C:\Program Files (x86)\Common Files\Microsoft Shared\Speech\Tokens\TTS_MS_ru-RU_Elena_11.0\MSTTSLocruRU.dat"
   "VoicePath"="C:\Program Files (x86)\Common Files\Microsoft Shared\Speech\Tokens\TTS_MS_ru-RU_Elena_11.0\ElenaT"
   
   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech Server\v11.0\Voices\Tokens\TTS_MS_ru-RU_Elena_11.0\Attributes]
   @=""
   "Age"="Adult"
   "AudioFormats"="6"
   "Gender"="Female"
   "Language"="419"
   "Name"="Microsoft Server Speech Text to Speech Voice (ru-RU, Elena)"
   "Vendor"="Microsoft"
   "Version"="11.0"
   
   After

   Windows Registry Editor Version 5.00
   
   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech\Voices\Tokens]
   
   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech\Voices\Tokens\TTS_MS_ru-RU_Elena_11.0]
   @="Microsoft Server Speech Text to Speech Voice (ru-RU, Elena)"
   "419"="Microsoft Server Speech Text to Speech Voice (ru-RU, Elena)"
   "CLSID"="{a12bdfa1-c3a1-48ea-8e3f-27945e16cf7e}"
   "LangDataPath"="C:\Program Files (x86)\Common Files\Microsoft Shared\Speech\Tokens\TTS_MS_ru-RU_Elena_11.0\MSTTSLocruRU.dat"
   "VoicePath"="C:\Program Files (x86)\Common Files\Microsoft Shared\Speech\Tokens\TTS_MS_ru-RU_Elena_11.0\ElenaT"
   
   [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech\Voices\Tokens\TTS_MS_ru-RU_Elena_11.0\Attributes]
   @=""
   "Age"="Adult"
   "AudioFormats"="6"
   "Gender"="Female"
   "Language"="419"
   "Name"="Microsoft Server Speech Text to Speech Voice (ru-RU, Elena)"
   "Vendor"="Microsoft"
   "Version"="11.0"
   
5. Save the changes and import the registry file by running it.
6. Restart Soundpad.