YAMMER is a product from Microsoft. It opens in it's own window (like a webpage) But with zero controls for disabled users. Unlike Edge, there is NO "READ ALOUD" feature! So I tried to use "Windows 10 Narrator". Unfortunately that product doesn't recognise TEXT in the YAMMER Window. Very poor! :-(
Thanks for sharing your feedback, we have a few questions about your post. 1. What is your Windows 10 build number? 2. Are there any specific keystrokes in Narrator you're using such as scan mode (Narrator Key (Caps Lock or Insert) + Space))? 3. Have you tried using the web version of Yammer?
You can also use this article here to learn more about how to navigate around Yammer with a screen reader https://support.office.com/en-us/article/use-a-screen-reader-to-explore-and-navigate-yammer-3462484a-bfd1-4d72-9cfc-0633b3322689. Please let us know if these resources helped you, and continue to leave your feedback with us.