When you are using your phone, you can see what is happening on screen and therefore do not really need to have reminders and notifications sounding. If you would prefer to keep things a little quieter, you might want to take a look at Shush. This handy free app can be used to keep your phone quiet when it is unlocked, but ensure that it sound alerts at other times.
You can grab a free copy of Shush from Google Play, and once installed you will find that it is very simple to use. Fire up the app and make sure that the option labelled Enabled Shush is ticked.
You may find that there are no customizations needed, but if you like there is the option of having your phone vibrate instead of remaining completely silent.
While the app itself is free, there is potential to extend its features slightly. Head to the Apps tab and there is an in-app purchase available that can be used to configure exceptions for different apps.
For the most part this is an app you can install and forget about. Whenever you are using your phone, notifications will be silent, but when the screen is locked they will sound as normal.