Running 6.28.1 (and prior) on Ubuntu 23.10 (and prior) I was never able to get the text to speech working well in the Viewer. It sort of worked, but was just about unintelligible. I spent hours on it and finally gave up.
Today I saw a post for Pied, a solution to install Piper and get it working with Speech Dispatcher. I gave it a try, and now the Viewer Read-aloud feature works perfectly for me. And I got several very pleasant voices. (Note, this is Beta, but I have had no problems.)
If other Linux users want to try it:
https://pied.mikeasoft.com and installation at
https://snapcraft.io/pied
The author is on Mastodon: @mikesheldon@octodon.social