This is how you increase the sound of vehicles (hearing mantas behind you etc...) and character sounds (footsteps/grunts etc...)

Open up UTEngine.ini in Documents\My Games\Unreal Tournament 3\UTGame\Config

Find: 

[Engine.AudioDevice]

You will see:

SoundGroups=(GroupName=Character,Properties=(Volume=X))

and

SoundGroups=(GroupName=Vehicle,Properties=(Volume=X))

Change the X to whatever volume you want.  For example- 8 for footsteps and 4 for vehicles.  This could be what k1 and Ex are using to get shots off us like they have radar (I havn't tested this yet so I'm not sure how much of an advantage it is).