Q: How can I add my custom songs?

A: Open Steam\steamapps\common\POSTAL2Complete\System

Find and open NicksVehicle.ini

[ProjectMaricaCore.NKVehicle]
CameraDist=15.000000
bFreeCamera=True
bDebugHUD=False
MeshRenderLevel=MRL_High
bAllowDynamicShadows=False
bBlockCorpses=True
NETAntiGlitchTime=60.000000
MaxDesiredFOV=105.000000
bWildRideFOV=False
bAllowProjectionLight=False
bAllowLight=True
MusicVolume=250.000000
bUseCustomRadioSongs=True
RadioSong=Sound'MaricaMusicPack.LAB_-_Beat_the_boys'
RadioSong=Sound'MaricaMusicPack.Scattle_-_Knock_Knock'
RadioSong=Sound'MaricaMusicPack.Megadeth_-_Symphony_Of_Destruction'
RadioSong=Sound'MaricaMusicPack.Onmyouza_-_Nemuri'
RadioSong=Sound'MaricaMusicPack.The_Ratt_-_Round_And_Round'
RadioSong=Sound'MaricaMusicPack.MegaDrive_-_Acid_Spit'

It should be TRUE
bUseCustomRadioSongs=True

Add custom song here
RadioSong=Sound'MaricaMusicPack.LAB_-_Beat_the_boys'
RadioSong=Sound'MaricaMusicPack.Scattle_-_Knock_Knock'
RadioSong=Sound'MaricaMusicPack.Megadeth_-_Symphony_Of_Destruction'
RadioSong=Sound'MaricaMusicPack.Onmyouza_-_Nemuri'
RadioSong=Sound'MaricaMusicPack.The_Ratt_-_Round_And_Round'
RadioSong=Sound'MaricaMusicPack.MegaDrive_-_Acid_Spit'
RadioSong=Your song ?

Q: So wtf is that? What is the sound'' ?

A: Read modding instructions(guides) by RWS, in the section how to add own sounds to game