So I have one notification sound that is ridiculously louder than all the rest. Is there a way to just adjust the volume on that one app notification sound while leaving the rest alone?
Welcome to Android Central! Which phone? You can't do that with the native sound control -- and I'm not sure if there's a 3rd party app that can adjust notification volumes for specific apps. Your best bet would be to select a different notification tone for that app that's inherently softer. Try all of the different preinstalled notification tones, and if you can't find one, then look for one online (like at zedge.net).
I agree , natively you can't, maybe try changing the notification tone of the app to something different might work as they have some loud and some lower tones .
If you have a computer, there are audio editors that will easily change the master volume for a sound file.
Copy the sound file from the phone to the computer.
Edit the sound file.
Copy the sound file from the computer to the phone.
I do this every now and then. Since I already have the editor program on my computer (Win10, Audacity), I have never searched for a similar app for my phones.