[LG G Pro 2] spit_fire:
So I got the G Pro 2 (D838) about three days back. Outstanding phone but the volume output was lower than I'd have liked it to be. Editing the snd_soc_msm_Taiko file like the G2 didn't do much. Downloaded the Viper audio app (good stuff, but couldn't increase the volume gain much). And then I stumbled on this thread of the G3 which did the trick: http://xdaforums.com/lg-g3/general/guide-headphone-speaker-volume-booster-t2819821

REQUIREMENTS:

1. Root

2. File Manager with Root Access (ES would do)

3. Text Editor (ES would do again; but if you're more comfortable copying the file to PC, something like Notepad++ would do)

STEPS:

1. In the root explorer, go to /system/etc and find "mixer_paths.xml". This is the file you need to edit. As mentioned, you can either do it on the phone using text editor, or copy to the PC and carry out changes there. Backup this file someplace before doing anything.

2. Find the following values and change it (as per your requirement). The below values have been modified to my current setting, and is

crazy loud

. My ears can only manage levels around 70-80%. So choose to increase values wisely:

[COLOR="SeaGreen"]<ctl name="RX1 Digital Volume" value="76" />
<ctl name="RX2 Digital Volume" value="76" />[/COLOR]

<path name="speaker">
<ctl name="SLIM RX1 MUX" value="AIF1_PB" />
<ctl name="SLIM_0_RX Channels" value="One" />
<ctl name="RX7 MIX1 INP1" value="RX1" />
[COLOR="SeaGreen"] <ctl name="RX7 Digital Volume" value="67" />[/COLOR]
<ctl name="SPK DRV Volume" value="7" />
</path>

<path name="headphones">
<ctl name="SLIM RX1 MUX" value="AIF1_PB" />
<ctl name="SLIM RX2 MUX" value="AIF1_PB" />
<ctl name="SLIM_0_RX Channels" value="Two" />
<ctl name="RX1 MIX1 INP1" value="RX1" />
<ctl name="RX2 MIX1 INP1" value="RX2" />
<ctl name="CLASS_H_DSM MUX" value="DSM_HPHL_RX1" />
<ctl name="HPHL DAC Switch" value="1" />
[COLOR="SeaGreen"] <ctl name="HPHL Volume" value="30" />
<ctl name="HPHR Volume" value="30" />
<ctl name="RX1 Digital Volume" value="67" />
<ctl name="RX2 Digital Volume" value="67" />[/COLOR]
<ctl name="COMP1 Switch" value="1" />
</path>

<path name="headphones-advanced">
<ctl name="SLIM RX1 MUX" value="AIF1_PB" />
<ctl name="SLIM RX2 MUX" value="AIF1_PB" />
<ctl name="SLIM_0_RX Channels" value="Two" />
<ctl name="RX1 MIX1 INP1" value="RX1" />
<ctl name="RX2 MIX1 INP1" value="RX2" />
<ctl name="CLASS_H_DSM MUX" value="DSM_HPHL_RX1" />
<ctl name="HPHL DAC Switch" value="1" />
[COLOR="SeaGreen"] <ctl name="HPHL Volume" value="30" />
<ctl name="HPHR Volume" value="30" />
<ctl name="RX1 Digital Volume" value="70" />
<ctl name="RX2 Digital Volume" value="70" />[/COLOR]
<ctl name="COMP1 Switch" value="1" />
</path>

<path name="speaker-and-headphones">
<path name="headphones" />
[COLOR="SeaGreen"] <ctl name="HPHL Volume" value="22" />
<ctl name="HPHR Volume" value="22" />
<ctl name="RX1 Digital Volume" value="57" />
<ctl name="RX2 Digital Volume" value="57" />[/COLOR]
<ctl name="RX7 MIX1 INP1" value="RX1" />
<ctl name="RX7 MIX1 INP2" value="RX2" />
[COLOR="SeaGreen"] <ctl name="RX7 Digital Volume" value="78" />[/COLOR]
<ctl name="SPK DRV Volume" value="8" />
</path>

3. IMPORTANT- Once complete, make sure the permissions are set to rw- r-- r--. This step is extremely important or you'd probably end up soft bricking your phone.

4. Reboot your phone and blow your head off.

DISCLAIMER: Not responsible if you bricked your phone, killed your headphone due to crazy increases, killed the neighbor, swallowed your phone, etc.

ALWAYS BACKUP BEFORE ANYTHING. Set permissions correctly and you'd be fine. Report back with ROM/ model no, and what not if this doesn't work.

PS. The Viper4android app does work after this; confirmed.