I upgraded to version 7.5.8.2 a few months ago. I am using ubuntu 20.04.  

In a nutshell Impress stops playing any and all sounds in a new session.

To reproduce
1.  Create a number of slides.
2. Add a transition for each slide and add a different wav file for each skide transition(for me the sound files are in a sub dir of the presentation). When adding each sound file use the Play button in the sounds Open dialog and observe the file plays no problem. 
3. Run the presentation and verify all sounds play.
4. Close Impress
5. Start a new Impress session, for me my workflow is that I close the presentation and shutdown my PC as its usually the end of the day.
6. Run the presentation, observe none of the sounds will play again even if you reload the same sound files from the same location yet incidentally the sounds still play in the open dialog.

Detailed description.
I'm working on a large presentation thst has 30+ slides each containing either a single sound from a wav file, which is added via a slide transition or slides containing several sounds in one slide each attached to a separate image transition, as you can appreciate it takes a long time to add all the individual wav files to transitions and once I have reached a milestome I run my presentation and everything works and all audio files are played, that is until I close and reopen the presentation and none of the files will play, I tested everything over and over, check my sound setting, file location were still accessible, checked the wav files played in multiple apps and found nothing to prevent the wav files from being played, once Impress stops playing the files from that point it will never play the files evr again no matter what I try, initially I thought it was perhaps an issue with the file names and assumed Impress had a problem with th file names which all contained hyphens, and I changed all the file names and replaced them with underscores, then I had to painstakingly modify every slide and reload all of wav files into each transition and incidentally while loading every sound file into each transition, via the Sound drop down list, I always test the sound using the very useful "Play" function in the Open file dialog and every file played from there, once I had changed the names it all worked again, however the next time I started my PC and run the presentation none of the audio files will play, confirming its nothing to do with the file names or the files or anything outside of Impress and therefore it appears to be a bug with this version of Impress and since ai couldnt find any sound related bugs I can only assume the bug persists in the latest version. I really need to complete this presentation and this is a show stopper for me personally,