found it ! dont ask me why but apparently popcorn doesn’t use your $PATH, it just creates its own: Add VLC installed via Flatpak on Fedora 36 by BaktashGorgani · Pull Request #2635 · popcorn-official/popcorn-desktop · GitHub
the “funny” thing is that it contains some nix paths but not /etc/profiles/per-user/teto/bin/ which is where vlc is for me (installed via home-manager). I added vlc to environment.systemPackages and now it finds it. The video I had no audio with popcorntime works fine when using VLC as external viewer. Problem solved.