Here's how you can turn off the in-app notification for repeat timers, and just hear the sound once instead.
By default, Timer+ always shows you a popup alert when your timers are done. With the "Show in-app notification" setting, you can make an exception and suppress the popup alert, while playing the sound once, for all of your repeat intervals except the last one.
So, for example, if you had a timer repeat 4 times, it would alert you like this when Timer+ is running:
When Timer+ is not running, the iOS notification system will always show you an alert and play the sound based on your timer's sound settings.