Hello bart271,
Thanks for using Apple Support Communities.
We understand you are experiencing some issues with using a third party app on your MacBook Pro. We are happy to help provide some information.
To start, we want to make sure that your MacBook Pros sound is working properly when using it. Are you able to hear sounds normally in other apps such as Safari and FaceTime?
One step you can take on your MacBook Pro is try booting in to Safe Mode to clear any system issues that might be preset that are causing this behavior:
How to use safe mode on your Mac
If you are only experiencing issues with the Bongo app, we would recommend troubleshooting the app from some of the steps listed on their support site, or contact them directly in regards to the behavior you are seeing. Take a look at their information here:
Bongo: Knowledge Base
Bongo: Contact
Best Regards.