Sound is a really hard topic. As more advanced, your setup gets harder to understand how audio works in detail. Whether you or your viewers are not able to hear the sound played by the alert, are the most common issues when it comes to audio.

You can hear the alert sound but your viewers don't / Your viewers hear the alert but you can't

Solution 1)

OBS Studio

Check if the OBS audio mixer of the browser source is set to “monitor + output”.
To get there you right-click the gear icon of your SoundAlerts audio source in the OBS audio mixer and press the “Advanced Audio Properties”

Streamlabs OBS

Check if the Streamlabs OBS audio mixer of the browser source is set to “monitor + output”.
To get there you right-click the gear icon in your “Mixer” tab and press the “Advanced Audio Properties”

Note: This depends on what you use as your standard and other monitoring devices. Could be your headset, speakers, voicemeeter, etc.

Solution 2)

OBS Studio

Open your OBS settings and go to "Audio". Check if your monitor audio output device is set correctly.

Streamlabs OBS

Right-click your Sound Alerts browser source and head over to properties. Choose your desired device and click on save.

Note: This depends on what you use as your standard and other monitoring devices. Could be your headset, speakers, voicemeeter, etc.

Twitch Studio

Please check if you have “”Enable Sound” turned on. If you don’t, turn that on and try to reload the browser source to make sure everything is updated.
Make sure to press "save" after you've changed your settings.

Note: If you are not able to see the Advanced Audio Properties, you might not have the sound alerts extension as a separate OBS audio source. Please take a look below to see how you do this.

Add the Sound Alerts extension as a separate audio source in OBS

To add the Sound Alerts extension as an extra controllable audio source, right-click your Sound Alerts browser source and select properties.
Put a checkmark at "Control audio via OBS".