Syncing via bazarr failed for 2 "old" movie

Subtitle sync failed for 2 movies (no speech found)

I'm running Bazarr on Qnap nas (Qbazarr) version V0.9.0.2, all other subs synced correctly so might be a general problem with the app ?

The movie audio is: .AVC.DTS-HD.MA.5.1

(sorry no tarball)

Traceback (most recent call last):
 File "/share/CACHEDEV1_DATA/.qpkg/QBazarr/bazarr/bazarr/subsyncer.py", line 46, in sync
  result = run(self.args)
 File "/share/CACHEDEV1_DATA/.qpkg/QBazarr/bazarr/bazarr/../libs/ffsubsync/ffsubsync.py", line 275, in run
  reference_pipe.fit(args.reference)
 File "/share/CACHEDEV1_DATA/.qpkg/QBazarr/bazarr/bazarr/../libs/ffsubsync/sklearn_shim.py", line 214, in fit
  self._final_estimator.fit(Xt, y, **fit_params)
 File "/share/CACHEDEV1_DATA/.qpkg/QBazarr/bazarr/bazarr/../libs/ffsubsync/speech_transformers.py", line 250, in fit
  raise ValueError(
ValueError: Unable to detect speech. Perhaps try specifying a different stream / track, or a different vad.

Start with bazarr/subsyncer.py and libs/ffsubsync/ffsubsync.py, then follow the traceback into sklearn_shim.py and speech_transformers.py. Reproduce the failure with the affected movie audio if available and determine what evidence is needed for subtitle syncing to complete without the "Unable to detect speech" error.