Well, I am German and I think I can help you how to pronounce it. It is funny, when I go to a gun shop in the US and talk about Heckler&Koch, I usually just say HK, since a get funny looks when I say Heckler&Koch the german way, Haha!
Well....Koch is the German word for "chef" or "cook". Check out his link. it is from a german English/German dictonary website and you can click on a little symbol behind the word to listen to a translation. The "Koch" in Heckler&Koch is pronounced the same way as it is in the example you can hear when you click on the symbol behind "Koch".
And by the way....you don´t even want to know how the local "Schwäbischen" (swabian) employees at the H&K Headquarters are pronouncing the name in their funny south east german swabian dialect.