Now, IP is a "Layer 3" technology and is used to encapsulate digital voice so that it can be transported. "Wireless" or, in the case of VoWLAN, "802.11," is a "Layer 2" technology. Therefore VoWLAN is simply Voice over IP over 802.11 wireless Ethernet. When people refer to regular VoIP it can be running over Ethernet, SONET, Token Ring, T-1s, whatever. The underlying transport doesn't matter and that's the simplistic elegance of the OSI model.
Net-net: VoWLAN is simply VoIP running on a wireless local area network. Someone slapped the acronym together because that is the nature of High-Tech – everything needs a catchy acronym.