Hello, in this guide I will help you enable 5.1 sound (7.1 should work but I am not fully aware), and this is not for people using Creative Audio Hardware since they have a tool called "Creative ALchemy" that already allows them to enable 5.1 with ease. This guide is for people with any other sound hardware, including onboard, but I do not know what hardware this won't work on; leave a comment telling me what hardware you have if it doesn't work so we can let others know. But, more importantly, it SHOULD work on almost all audio hardware.
An important tip: the replacement audio file also works for just about any game that is older with broken surround sound, such as STALKER games, and it's a matter of using this new file and renaming it to replace the old one.

Let's get going, this will all be a very simple process with no convoluted dependants or chance of wreaking havoc on your computer or the game, so don't worry. First, please open up your Killing Floor system folder: \steamapps\common\KillingFloor\System - we are going to only change 2 files here.
Second, let's rename "DefOpenAL32.dll" (it might not say .dll at the end for some people and that is okay) to something like "Null_DefOpenAL32" so you have this file on hand, because why not.
And after that, then open "KillingFloor.ini" with a notepad program, and go to the area
"[ALAudio.ALAudioSubsystem]" and we are going to change the lines under it, such as, "Channels=32" to equal, instead, 128 "Channels=128" (set this to '64' if you are using older onboard sound like Realtek, like before 2012") and then, lastly, we are going to change "TimeBetweenHWUpdates=15.000000" to "TimeBetweenHWUpdates=1.000000" - this will fix a problem with lots of missing sounds, save the changes and exit.
The second tweak for "KillingFloor.ini" is necessary, and the first is a nice luxury that easily handled nowadays.

Now, we need to download a newer sound driver for the game, that supports surround sound, which is - http://openal-soft.org/openal-binaries/openal-soft-1.19.1-bin.zip (if this newer 1.19 version gives you problems than use this older one) - http://kcat.strangesoft.net/openal-binaries/openal-soft-1.17.2-bin.zip (there are no ads, installers, or unrelated files from this) - and once that is downloaded, please open it and navigate to: /openal-soft-.1.17.2-bin/bin/win32/ - and click the file "soft_oal.dll" and drag it into your killingfloor/system folder and place it in there. Then click this new file inside the folder and rename it to "DefOpenAL32" to take the place of the old one (Do not use the win64 version of this inside the archive NO MATTER WHAT).
Now, run the game and go into options>audio and make sure 'System Driver' is unchecked and some form of 3D Audio is enabled, you will probably want 'HW3DAudio + EAX' unless your sound stops working and then the safe bet is to put it on "3D Audio".
And, that is it!!!

I hope you get to enjoy surround sound now, and if you are pleased with the results then I would ask of you to 'favorite' this and 'rate up' - those 2 things are very very important and almost no one does it and you would be very helpful if you chose to be that person. You're Welcome!