Thanks for your review!

* I've tested the extension under Ubuntu 18.04 LTS and it works fine. Unfortunately, I don't have an Arch system for testing.
* It looks like it's not allowed for web extensions to access locally stored files which makes it difficult to implement an option for choosing a sound file. I'm open for any tips on this topic because I'd like to see this feature as well.
* Unfortunately, Firefox doesn't allow web extensions to open the about:downloads page at the moment.
* Since version 1.4 it's possible to setup a different text for failed downloads. Default is "Download failed". I'm thinking about adding a different sound for in the next version.