Podcast not playing in Ubuntu 18.04 LTS

Prerequisites

[x] I have searched the Issues list for open and closed issues and believe this is a unique, currently unreported bug.

Describe the bug

The sound of the podcast won't play in ubuntu 18.04 LTS.

Everything else in the application works as intended in my machine; however, when I go to play a podcast it doesn't play.

To Reproduce

Steps to reproduce the behavior:

Go to 'flatpak run com.github.needleandthread.vocal/x86_64/stable'
Click on 'Any podcast'
Scroll down to '....'
See error: Podcast won't play

Expected behavior

Reproduce the podcast

Screenshots or screen recordings

Logs

vocal

Platform Information

Please share as many of the following details as possible:

Linux distrubution: Ubuntu
OS version: 18.04 LTS
Desktop environment: GNU

Please check the applicable option below:

[ ] I'm using the latest version from git that I've manually compiled
[x] I'm using the latest version of the Flatpak, downloaded from Flathub.org
[ ] I'm using the latest version available from AppCenter
[ ] I'm using a different version (please note here) :

Additional context

I've tried downloading a podcast and then going to ~/.local/share/vocal where the podcasts are stored and I opened the file with VLC player and it plays fine, so the problem must be in the media player.

I've tried uninstalling, resetting (as shown in FAQ) and installing it again from flatpakhub but it doesn't work.

Also, the very first time that I opened the app (first install using flatpak run com.github.needleandthread.vocal/x86_64/stable) it worked fine. Then, it showed up as an app in the Ubuntu "Show Apps" list and that's when it stopped working.

I hope it's just an issue of mine as the applications is beautifully built and I appreciate it.

Full log

This is the log I get when I run the app and play the first podcast I see vocal-1 vocal-2 vocal-3

VocalPodcastProject / vocal