If you cannot find Background Apps, try searching for "Installed Apps". To check the details of the apps, click the three dots and select 'Advanced options'. See if you can disable the sounds.
However, if this doesn't work. Open the volume mixer, to do that, right-click the speaker icon in your system tray and select "Open Volume Mixer". Check if any specific apps are making sounds even when their system-level notifications are off. You can mute individual apps here.