Have a look at the Accessibility page of the Tails documentation.
It appears that accessibility options aren't activated in the welcome dialog, but instead from the universal access menu.
From the linked page:
You can activate most of these technologies from the universal access
menu (the Universal Access icon which looks like a person) in the top
"Most of these technologies" includes the Orca screen reader, though there's a small caveat:
The screen reader does not work with Tails Installer nor with the
Unsafe Web Browser and I2P Browser.
For the day-to-day use of Tails, this caveat shouldn't pose a problem:
- The Tails Installer is only used once, when creating a live USB stick;
- The Unsafe Browser is different to the browser that a user would use most of the time - most of the time they would be using the main, "safe" Tor-ified browser;
- The I2P Browser is also different to the main browser, and only used for I2P traffic.