Google calendar event reminder sound is too short and I can't hear it
Go to "My Files" and then copy any mp3 or sounds that sounds longer to be your reminder. Then go to "device storage" look for "notifications" folder and paste it there.

Now go to your S Planner and go to "Settings" now you'll see your mp3 as the choice in the notifications tone options. Now you'll have a longer reminder tone 
Hope this will help