I’m using the Google TTS backend for my Mark 1, and I wanted to change the voice; I’ve now done so, so I thought I’d write down how for future reference. This is pretty heavily technical, so if you’re not comfortable at that level, don’t do this; wait for the Mycroft team to make it easier 
First, this only works for the Google backend (Mimic has its own ways of changing voices). So, to use the Google backend, on your Mycroft web config screen, choose Settings > Advanced and set “Google” rather than “Mimic” as the Text-to-Speech Engine. (Note: don’t do this if you’re not comfortable with Mycroft’s responses to you going via a Google service; Mycroft uses Mimic by default for a reason.)
Once that’s done, you’ll get responses from Mycroft using the default Google voice, which appears to be American. You can change this by adding a manual config setting. SSH into your Mycroft: you will need to enable this, and the documentation explains how to enable SSH on the Mark 1. If you’re using an Ubuntu machine (and possibly others) then your Mark 1 should be available as mark1.local, meaning that you can just do ssh pi@mark1.local and log in with password mycroft; if not, then the docs above explain how to find the IP address and connect. Once connected:
sudo su to become root, then su mycroft to become the mycroft user. Edit the config file with nano ~/.mycroft/mycroft.conf; it’s a JSON config file (the docs explain Mycroft config files). If there’s nothing in it, then you can add: {"tts": {"google": {"lang": "en-uk"}}} and save and you’re done. If you’ve already customised this file, you’ll need to add that key to what’s there. Fortunately, this config file only overrides the stuff that’s in it, so it’ll pick up the other keys from the web configuration and then override them with your local changes. You can choose a voice other than en-uk; the voices available are listed in the gTTS repository at https://github.com/pndurette/gTTS/blob/master/gtts/tts.py#L18.
Once you’ve made that change, you need Mycroft to notice that you’ve done so. You can just restart Mycroft, which works fine. To be more efficient you could just restart the mycroft-speech-client process.
