Short version: Gaupol can load a video file but freezes when loading an audio file. Perhaps I'm just missing a required package?

This is on Debian Buster with Gnome using these packages:

ii  gaupol                                1.5-1
ii  gstreamer1.0-gtk3:amd64               1.14.4-1

How to reproduce:
Firstly get a valid video file (which Gaupol will be able to load ok), then convert this to mp3 (which Gaupol will fail to load properly):

wget https://download.blender.org/durian/trailer/sintel_trailer-480p.mp4
This gives a 4.2 MB video file looking like this:

sintel_trailer-480p.mp4: ISO Media, MP4 Base Media v1 [IS0 14496-12:2003]
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 854x480, 537 kb/s, 24 fps, 24 tbr, 24 tbn, 48 tbc (default)
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 126 kb/s (default)

VLC can of course play this video no problem.
Start gaupol from the command line, "File -> New", "Video -> Load Video" and select the downloaded file "sintel_trailer-480p.mp4". Video starts playing immediately. Great.
Close gaupol.

Convert this video file to mp3 using VLC's "Media -> Convert / Save", using the profile "Audio - MP3". This gives an 812 KB mp3 file "sintel_trailer.mp3". Again, VLC is able to play this mp3 audio file.

Start gaupol from the command line, "File -> New", "Video -> Load Video" and this time select the audio file "sintel_trailer.mp3". Gaupol appears to freeze, doesn't show video and doesn't respond to menu commands any more. "top" reports gaupol only using around 1% of CPU but the window doesn't respond. After some time a popup says "Gaupol is not responding" with two buttons, "Force Quit" or "Wait". Nothing is shown on the console apart from "Killed".

Workaround:
Converting the audio to ogg or aac makes no difference, the only workaround I've found is to use ffmpeg to convert the mp3 back into a video file using a still image, and then gaupol is able to load the video as before.