Hi,
Unfortunately, at this time, the specific demo app source code in the WWDC session is not pending public release.
You can however copy from the "Code" tab in the Developer app. It has the basic implementation skeleton to get you started.
If you already have a PTT or VoIP calling app, you should be able to reuse the vast majority of what you already have.
See also https://developer.apple.com/forums/thread/707689
This is what someone at Apple posted in another thread.
tudorg
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Also received this, I don't have any paid content in any of my apps.
Also not using iTunes Connect nor Apple Music.
Just the other day I received another one about Apple Books (also don't have any published content on Apple Books):