Does the audio issue only occur within the AT&T app or to all channels installed on your Roku device? If it's just on one channel, removing then re-installing it may help. To ensure the process is successful, make sure to restart your Roku before adding the channel back in.
I spoke with AT&T support. We troubleshot the issue. I updated and re-installed the AT&T app. All the other apps work. I subscribed to YouTube TV instead of AT&T TV due too the problem. YouTube TV sucks. The audio is mono PCM (only) no (5.1 sound). The Amazon and Plex apps work fine I'm getting 5.1 sound via Roku. I purchased a Roku Ultra. I abandon the Roku stick. My Yamaha 7.2 system is useless with the YouTube TV app. The problem with the AT&T app started right after the Roku June 30th 2021 update. The update broke the Roku stereo sound.