Is the DLC included with the base game?
If it isn't, add the keypoint back and add a 
If you buy the new game, then it's all there, but if you have original version without the DLC you can't buy them seperately anymore, meaning you have to buy the whole GOTY edition to get the DLCs.
As the game is pretty old and only places selling regular version are retails and third party key resellers mostly, I guess it should be mentioned under DLC instead.
Distorted Audio Fix
I've been following discussion on the Steam community and I'm about 100% confident that the only resolution for the 7.1 distorted audio problem is to set the audio device to 5.1 or lower. I've tried the fixes involved with modifying the Audio.scr file and that doesn't seem to be a solid solution. I've seen others mention that modifying the Audio.scr file doesn't seem to work 100% for them either. For me, if I restart the game enough, the audio might work correctly one time out of twenty so you might change Audio.scr and then get lucky and not have the problem after starting the game but then have the issue show up the next time you start the game. People have mentioned using Virtual Audio Cable (not a free program) as a solution and I finally looked into that. Turns out that Virtual Audio Cable basically creates a stereo sound device that you can route to the Logitech headphones; Dead Island sees that stereo sound device and then outputs in stereo. This is a solution for Logitech users because Logitech USB headphones don't seem to have any speaker configuration option other than 7.1; they can't change their device to 5.1 or even stereo since the option isn't available in the Windows speaker config (see this link)!