The last thing people may do is fuck with their sound volume.  Obviously you can control your sound volume in game but people tweak their INI for more control over sound groups that aren't available in game.  This has already been discussed in two different threads:

http://www.dfbgaming.com/Thread7211.html
http://www.dfbgaming.com/Thread6729.html

But just to add here's a video of how people with tweaked sounds can hear vehicles/footsteps far away.  Sounds are actually muted if they're TOO far away.  Notice how the audio just all of a sudden is cut out once the viper is too far away.

<embed src="http://hosting.dfbgaming.com/members/emac/player.swf" width="720" height="480" allowscriptaccess="always" allowfullscreen="true" flashvars="file=http://hosting.dfbgaming.com/members/emac/Movie_0001.flv&amp;amp;autostart=false"></embed>

The main groups you want to focus on are Character (footsteps, grunts), Vehicle, Item and Weapon.