1. Adapt Sound [NO ROOT]
Adapt sound is a cool tool improving your sound quality in music and even in-call quality. The set.up is easy. The app will play you "beeps" and continuosly ask you if you can hear it. On the basis of this, the app can customize and improve the entire quality of sound.
Unfortunately, as the app is ported from the Galaxy S4, the density is set to 5´ display so you have to change it. (Special module for Xposed Installed can do it or, if you want, you can change the density of entire system by modyfing the .ro.lcd.density - application ROM Toolbox and others can do this)
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Another app, working only on older devices with older software. It comes as a paid app from the Google Play store. May not work on certain custom ROMs (CyanogenMod should work properly).
3. NexusLouder for Nexus 4/7/GalaxyNexus
NexusLounder works only on Nexus 4/7 and Galaxy Nexus. It generally boosts your phone's ringtone, notification, speaker volume and in-call volume. In short, cool thing.
Flash via recovery.
4. Beats Audio
Another cool stuff like this. In this time, it's the Beats Audio. As you maybe know, the HTC Corporation is working with the Beats company and puts the Beats' audiochips to Htc phones to make the sound better. Now, you don't have to buy the Htc, just flash the zip and you're done :).