Why is audio not playing when mirroring Substack and Threads from iPad to Roku Stream Stick?

Mirroring from my iPad to my Roku Stream Stick plays video and audio for sites like Netflix, YouTube, etc. But it will only play video and not audio when trying to mirror sites like Substack and Threads.

I've searched all over for settings that might remedy this, but so far have found nothing.

Any suggestions would be greatly appreciated.

[Re-Titled by Moderator]