Two notification sounds simultaneously in Debian+KDE

Hello,

I’m using MX Linux (**Debian**) with **KDE** version 5.27.5.
Slack version 4.43.52 is installed from Flathub.

Every time I receive a new message (or when I trigger a test notification in Slack's settings), I hear **two notification sounds simultaneously**:
- The standard Slack notification sound, as configured in Slack's settings
- A standard KDE Plasma workspace notification sound

It seems that Slack is sending notifications via D-Bus, which then triggers the system notification sound.

Unfortunately, in `KDE Settings -> Notifications -> Application Settings -> Slack`, there’s no option to disable sounds or fine-tune notifications for specific events. The only options available are shown in the screenshot below:


Could you please advise me on how to fix this issue?
Thanks in advance!