In addition to the iOS 15.1 beta yesterday, Apple also released a HomePod 15.1 beta that brings Lossless and Dolby Atmos Spatial Audio support to the HomePod and the HomePod mini.
The HomePod beta software is downloadable by invite only and developers do not have access to it for testing, However, 9to5Mac was able to confirm that Lossless and Spatial Audio support are included in the beta.
Apple added Dolby Atmos with Spatial Audio and Lossless support to iOS and Mac devices back in May and promised at that time that the feature would be coming to the HomePod and HomePod mini at a later time.
Lossless Audio and Dolby Atmos can be enabled through the Home app. To do so, do the following:
Note that these options appear in the iOS 15.1 beta, but the HomePod 15.1 beta seems to be needed to actually use the features on the HomePod.