Have you tried this? Open Gmail, then hit the Menu button and select More then select Settings. Under Account Settings select your account, then scroll to the bottom and select Labels to Notify. When you select, say Inbox, there will be more notification options.
I have an S5 Active. Is there any way to lock the Sound control on? Something keeps changing it to Vibrate or Off and I want it to stay ON except in Plane mode or "Timed Silence" app.
Second, I'm not aware of anything that will lock the sound, but if something is really turning it down on you without your knowledge, it might be a good idea to start with trying to figure out how or why it's turning down. I've never had a phone switch to vibrate or off without me pushing something or downloading an app that can control the volume and giving it permission to do so....