EAX is a sound engine that permits for multi channel audio and support things like reverb and other environmental effects. This is a proprietary engine, thus the need to license it from Creative to permit for support in other party's sound card, with any game that makes use of engine. This in and of itself is not a huge problem. It is a company making an innovation, and capitalizing upon investment they created. Particularly in some games, this makes it simple to add multi-channel audio and effects without creating your own sound engine.
The problem is that it restrictions customers who buy your games to a specific part of hardware must they want to experience your work to fullest. That is like a game being created that only permits AA/AF to be turned on using ATI or nVidia's graphics cards, as opposite to using a standard that both can utilize.
The other example is use of Pixel Shader standard; together leaders know what is necessary as far as hardware to run those kinds of programmable graphics, but whether they select to apply them is another matter. But API is available to them, or any other graphics company like Intel or Matrox.