Thanks for the feedback.
As I'm using the default Android mechanisms for posting notifications, it should already keep quiet if you put your phone into silent mode. Some manufacturers may have implemented the silent mode differently - or even offer different additional modes like a do not disturb mode.
Please check if your phone has something like this or if there's a setting that you need to add my app to in order for it to be silenced too. If that's not the case, you might consider activating the silent mode in my app (Settings -> Notifications -> Silent mode). Please note that you need to own the pro version to activate it.
I'd also be interested in what phone you use (brand, model, Android version).