hello,

since few time (1-2 months I think) I have a very strange bug where slack sound notification is "muted" on my system (the visual notification appears but no sounds).

After some investigations I successfully isolated the cause of the problem:
  - with my current firefox profile but with plasma integration extension disabled, sound works on slack notification.
  - with a fresh firefox profile but with plasma intergration installed and enabled, sound does not work on slack notification.

STEPS TO REPRODUCE
1. Run a fresh firefox profile
2. Install plasma integration extension and restart firefox
3. Connect to slack and go to preferences > notifications
4. Click on different sound to generate a notification

OBSERVED RESULT
visual notification appears (bubble info) but no sound at all


EXPECTED RESULT
the

SOFTWARE/OS VERSIONS
Linux: 5.3.8-arch1-1
KDE Plasma Version: 5.17.2-1
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.2
Firefox: 70.0.1
Plasma integration extension: 1.6.1

ADDITIONAL INFORMATION
while the last version of firefox extension seems prior to when the bug appears (little more than one month I think) and the last version of plasma I think this is due to a recent change on plasma desktop environment and especially the new notification system appeared in 5.16 version.

I am available to make any tests or feedback. Thanks.