This app will change the way you think about sound (seriously). Every time I demo the difference to someone by turning it on and off they immediately ask me to install it for them.
Installed using directions by @docnok63, paraphrased below for your convenience. I have only tested this app on my rooted T-mobile S5, and only with spotify (it's the only player I use). Of course you should always backup first and only you are responsible for what you do to your device.
1. Ensure you are rooted. From this point on, grant all superuser requests.
2. Download ViPER4Android FX. Get the latest version under the "Android FX" heading.
3. Extract the 4.x .apk from the zip. If you haven't already, smart-install Busybox from the play store.
4. Move the 4.x .apk to root system/app using a root-friendly file browser of your choice. Reboot to reset permissions.
5. Launch the app (from your app drawer). Grant root to let it install drivers. I've always gone with super audio quality; I suppose it's up to you. It may ask you if it can modify some files to disable power saving...just let it happen. Reboot again.
6. Go straight to settings/sound/music effects (at the bottom) and toggle Viper4Android as default.
7. Open root system/build.prop using a file explorer and text editor with root access. You change three lines of code as follows. Some may already have been changed.
Ipa.decode=true should read Ipa.decode=false tunnel.decode=true to tunnel.decode=false lpa.use-stagefright=true to lpa.use-stagefright=false
You can explore the main app thread here for some ideas on customization.
All credit due to @viper520, @zhuhang, and @docnok63.
All I did was paraphrase. Hope this helps someone!