I spoke with a.borque a while ago and resolved the issue, here's what I found.
MONSTER.SOU files contain two chunks per sound:
- VCTL - contains lip synch data (also sometimes VTTL)
- SOU - contains the Creative VOC file data
The German Sam & Max MONSTER.SOU seems to have a VCTL without an associated SOU entry.
I wrote a small tool to fix the MONSTER.SOU file - it will create a new MONSTER.SOU file and only copy data for each VCTL chunk that has a matching SOU chunk.
Perhaps a better solution would be to update the ScummVM tools to skip invalid sound entries.
Python source code