I've created an MP4 4K video and it has sound. However, I muted the sound from the video, and used the MP3 from Audacity that was recorded with a quality microphone. (Blue Yeti)

The MP3 works just fine on its own, as does the MP4. When I add them both to Kdenlive, mute the video's embedded audio while keeping the MP3's audio working, I can hear it just fine.

Next, I save the project in Kdenlive's format, and then I reopen the file. No audio plays from the same MP3. If I unmute the audio embedded in the video, it plays the embedded audio without issue.

Regardless of if I created the project from scratch in the same session, or if I saved and reopened it, Kdenlive does not save the sound to the exported MP4.

Just an FYI, I saw the multitude of issues with libmlt6, and I'm using the latest version.

apt-get: `libmlt6 is already the newest version (6.6.0-1build1).`

I installed `http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu bionic InRelease` to grab Kdenlive, and before that, I used the built-in repos that came with Linux Mint to install Kdenlive 17.x which I upgraded originally hoping it would fix the problem.

FYI, I'm using the Cinnamon desktop environment in case it matters.

Let me know if you need any other info to help fix this issue.