Known issue with tvOS 13 BETA Apple seems to always break things during early betas. Seems Atmos is one of those things.
I highly recommend that users resist the temptation to install tvOS beta's. On the AppleTV4K, there is no USB port so it is impossible to revert back to any other version. You will have to live with any issues until the final release. Because of this and other reasons, I don't install beta's, doing so breaks our ability to fix/test and push out updates.
As tvOS 13 is a major bump in version, I would expect there are lots of changes going on under the hood. tvOS 13 release is sometime in the Fall of 2019. 3-4 months away.