At least in Apple Podcasts, if you play a PeerTube video it's audio by default with the video playing in a tiny preview. If you click on the video it expands to the full screen, but you can just ignore it. You can even lock the phone and it carries on playing just the audio.
(But, of course, other podcast apps may behave differently.)
One thing that may help is PeerTube's roadmap plans to separate the audio and video streams in 2024, which might allow audio-only subscriptions?