Hello! I have the same problem in SLOBS. Alert sounds do Not work when pressing "Test widgets" or while streaming.
Do you get sound when you have the alerts page open in chrome?
Also, I do not get the sound when I have alerts page open in Chrome. However, I can hear text-to-speech, hear the alert sound when choosing sound media file and I am able to see the alert image. This goes to all the alerts (subs, follows, donations etc). I have reinstalled SLOBS twice now, deleted cache, reset API, removed all widgets and added them back and also tested this in another PC (windows 7 and windows 10). I have latest version SLOBS and all drivers are up to date as well.
Can anyone help me please?