Where to set default TTS voice?

Setup, Configuration and Use Runtime
2

Sorry, Karel, I indeed still need to document this somewhere.

What you need to do is to add this to your services.cfg:

org.eclipse.smarthome.voice:defaultVoice.<ttsId>=<voiceId>

e.g.

org.eclipse.smarthome.voice:defaultVoice.mactts=Anna

As you can see, you can define a default voice per TTS, so you can have multiple lines for different TTS.

Hope this helps!