Some linguists put this under "tones". Although you learn in every textbook that Vietnamese has 6 tones, in reality it has about 8 or even nine, depending on the scientific definition and view. Therefore it is hard to explain at the moment and for me, because I am not a linguist. There are certain phenomena, where the sound is stuffed in the mouth. Actually, Vietnamese do not say "m", they really say "ng" and stuff the sound, so it sounds like an "m". The same is seen with the snail (oc), which sounds like "op" - you pronounce a "c", but the air is stuffed, so it really comes to a "p" sound - at least for us foreigners. Anyway, it is wrong to say "khong" is pronounced "khom". Vietnamese clearly differentiate between the bilabial "m" and the "m-sound of ng". There is a short intro on this topic in Thomson's Vietnamese grammar, in the part of phonology.

I think the best way to know how to pronounce it is listening to the template pronunciation and imitate it. Otherwise, if you can read the IPA, you can follow this: http://vi.wiktionary.org/wiki/kh%C3%B4ng And you have to know that Vietnamese pronunciation highly depends on dialect.