A great talk on Flatpak sandboxing was released in March. [1]. The original post is from 2018 (Flatpak 1.0). Not sure if these issues are completely fixed yet, there are a lot of release notes up to version 1.63 (stable). [2] It seems that the maintainers and community are working on improving security continuously. My main hope is that they optimize for privacy, security and user control of the packages. Overall I'm impressed by flatpak with it's cross-distribution packaging.
I found it through search on Youtube, had been looking for which way to package applications on Linux. Packaging really takes a bit of time, reading documentation and figuring out how to do it. After packaging a flatpak you need to add it to a local repository on your device before you can test install it, which isn't as intuitive as clicking on a .appimage file, but brings some benefits. The documentation is quite readable: https://docs.flatpak.org/en/latest/first-build.html.
Agreed for audio quality. It's hard to listen to with only the left channel working. It might be worth reuploading it with balanced stereo.
[1] https://www.youtube.com/watch?v=3rCIEzfZw1I
[2] https://github.com/flatpak/flatpak/releases