Looks like this regressed in c1f5d8d023 (blender 3.1), basically
since then if there was no video, then no audio was ever written
either.
From what I can tell, the original change tried to fix the problem
that "file size autosplit" logic was after video, but before audio
data writing. So it moved audio writing to be before the split (good),
but also (not sure whether by accident) moved audio writing to
only happen if video is written.
Pull Request: https://projects.blender.org/blender/blender/pulls/132874
f355683e4b