[G4 General] Usraek:
So, after much tinkering I've figured out the definitive way to boot your headphone volume, that can likely be applied to other things such as the speaker and the handset volume.

***MAKE BACKUP OF ANY FILE YOU MODIFY - A BACKUP METHOD IS INCLUDED BELOW***

What you need:

Rooted Phone

FX File Explorer

FX File Explorer ROOT ADD-ON

5 minutes

1. Download and install the apps linked above. I chose that file manager because I've confirmed through a lot of use that it will not truncate your files leading to a bootloop and it does not modify your permissions when editing the file.

2. Open File Explorer. Choose 'System (Root)', grant access, then navigate to 'etc'.

(/system/etc)

3. Tap the overflow dots in the upper right of the screen and enable 'Mount Read-Write'. Click through.

4. Scroll down to your 'mixer_paths.xml' and long press on the file. Select 'Archive' and confirm. Do not change the type, as the Tar/BZip will leave your permissions intact. This is your backup.

5. Tap the 'mixer_paths.xml' file to open in the text editor. You can pinch to zoom in and out to make it easier to see if you've already changed your DPI settings.

6. Tap the edit button in the upper right. Once in edit mode tap the pencil icon in the upper right. Use the magnifying glass to find and search for 'lg rx device' (without quotes).

It would seem that the values under this header are the ones that actually do anything for our phone, though I've only tested the headphones myself.

7. Scroll down from 'lg rx device' until you find <path name="headphones-advanced">

8. Once you are there, you can modify 'RX1' and 'RX2' volume values to boost them. I have mine set to '94', though you may want to try different values yourself to compensate for distortion... Perhaps a bit lower. The default value is '80'.

(I would definitely suggest playing around with this, because with certain music genres or settings on your equaliser/Viper the distortion can range from not bad to absolutely horrendous.)

9. Tap the overflow dot menu in the upper right corner to save the file.

10. Reboot your phone and enjoy!!

This can also be combined now with the Xposed module 'Volumesteps+' or 'LG G4 Tweaksbox' for finer audio control. Not to mention Viper4Android and Poweramp.

V4A with Poweramp guide here:

http://xdaforums.com/showpost.php?p=62107040&postcount=109

Props to [MENTION=4018499]phillymade[/MENTION] and [MENTION=2861395]jesssiii[/MENTION] for spurring me to dig into this.

(Note: There are also 'HPHL' and 'HPHR' volume values, that I believe should control your individual left/right headphone volumes. I haven't tested, but based on what I've seen elsewhere in the Nexus 6/G2 forums you could probably try altering those values *instead* of the 'RX' values. This is a really good starting point overall for those who like to tinker.)

EDIT 04/FEB/2016:

Just followed my own tutorial again. It all functions on 5.1 with an H812. This time I set my RX volumes at '89' and my HPHL/HPHR at '21'. They seem to function independently just fine, so you can try one or the other. I tried higher values on each and the distortion is just gross. Even on these numbers I may need to tweak again so I can comfortably set volume on max without distortion worry. It really does depend on the track too, as there's less range needed with some genres.