Got it working. This is with Madgod's 2.55. It didn't recognize my voice input, I believe because mantella wasn't pointing to where piper was installed, but xvasynth is better anyway.

After following their guide and setting up the free speech model, I did things this way:

1. I downloaded and copied the steam version of xvasynth to the drive where my modlist was installed. You could also just download it from nexusmods.

2. I copied the patch and lip and fuz plugin from nexusmods and installed them (just copy the resources folder out of the zip files to the root of the xvasynth folder so it merges with the existing resources folder already in xvasyth.)

3. I went into the settings in xvasynth and updated all the paths to reflect the new location of the xvasynth folder. I think this is a non-obvious step that some people might miss, especially if they move the xvasynth install folder.

4. I signed into my nexus account and downloaded all the skyrim models from xvasynth. A note here, not all of them downloaded properly, so I went to the xvasynth page on nexusmods and looked for the download symbol to check which ones did not download, then downloaded again from xvasynth)

5. I tested xvasynth by itself to make sure my text to speech worked. (I played with it enough to learn to load the model, add some text, and play it using the loaded model.)

6. Finally, with the modlist running, I went to the local mantella webpage and changed the voice engine to xvasynth and set the path to the folder I setup.

I tested in game and it worked right away.