n01 I've already found the setting to choose the path, but available folders don't include the one I would like to save files in, I can see only a restricted amount of folders.
I've just checked that setting and I see what you mean. Only the Android folder is actually available (although there's also "data" and "media" subfolders, not just "obb"). A quick look at the Voice Recorder github shows this is apparently an issue with GrapheneOS. Very strange.
I think Recorded could play files too, so It reads them.
Yes. For recordings it creates. Apps can always access files that they create, but they need permissions for files they haven't created hence the app not needing permissions. I should have specified that before.