Update/correction:
The WAV file HAL has provided with v2.1 does not open in eac3to, but does open in mkvtoolnix - after freezing the program for a few minutes.
I just clicked “multiplex” and it seems to be taking forever but working. Will update this post when it finishes (or crashes).
JEDIT: Muxing failed with mkvtoolnix.
So converting the .wav is a must? I just downloaded MKVToolNix GUI and googling tutorials, haha! I can’t understand why adding an extra audio track in a video file can be so difficult.