This app was mentioned in 10 comments, with an average of 1.00 upvote
There's a good app out there called headphone equalizer It can identify which device is connected on bluetooth and apply a specific equalization for that set. Can also detect music genre and change the eq for that. If you use a few different sets of headphones it's well worth it.
The best app is Vuper4Android, but it needs root and it is not worth it. Are you using Bluetooth headphones? There are some apps in the Play Store, try THIS ONE or THIS ONE. I am still using my Note 8 and the Bluetooth headphones I have are quite good for its price. I tested them with songs I know how they should sound.
https://play.google.com/store/apps/details?id=com.manythingsdev.headphonetools
This app is a pretty easy one to adjust and seems to be working ok so far with the buds. Once it kicks in you'll hear a difference immediately. Whether or not you think it's an improvement sound wise is subjective I think, though. I think the audio is fine as is, but if you want more bass on heavy hip hop or something, then maybe add bass.
Maybe something like
https://play.google.com/store/apps/details?id=com.manythingsdev.headphonetools
Try this one https://play.google.com/store/apps/details?id=com.manythingsdev.headphonetools&hl=en
Otherwise, I would just resolder minijack changing right and left cables.
I think this app does what you're looking for.
https://play.google.com/store/apps/details?id=com.manythingsdev.headphonetools&hl=en this one works great.
Louder? Sounds like you could use new headphones. Usually maximum is insanely deafeningly loud. Anywho, try this app: https://play.google.com/store/apps/details?id=com.manythingsdev.headphonetools