I called last week to get an answer on this. The case got escalated and I received a response today.

Initial email:

Hello,

Thank you for taking the time to contact us. The STARZ app provides a stereo mix. This would give you left and right audio but it would not give you surround, which is what is seems like you are asking about. We are working on providing a surround mix in future builds.

Regards,

-Jason

I replied, asking if I was crazy or if the app used to support 5.1 (I thought I remembered it supporting it, being the only 1 of the big 3, Starz, HBO and Showtime). I received another response:

Hello,

Thanks for the response. The app did have 5.1 in the past on Android TV. A change in the OS caused us to move to a stereo feed until an underlining issue is resolved. I do not have a timeframe on when that will be resolved and when 5.1 will be available again.

Regards,

-Jason