Analog Devices recently published a document (PDF) challenging reviewers not only to benchmark the CPU utilization of integrated audio solutions, but also to explore how well those implementations handle EAX positional 3D audio. According to the document, most motherboards don't handle EAX properly, despite the fact that they claim to support the standard. Analog Devices diplomatically avoids naming names, but with the vast majority of integrated audio solutions powered by Realtek codecs, the writing's on the wall. TechReport has rounded up a couple of nearly identical motherboards with Analog Devices and Realtek codecs to see how they handle EAX positional 3D audio, with conclusive results:

There are clearly problems with the EAX implementation of Realtek's HD Audio codec and its associated drivers. Despite the fact that Realtek claims to support EAX 2.0, obstructions and occlusions apparently haven't been implemented correctly, if at all. For some games, that's not a serious problem. For instance, FEAR plays fine, even if the audio isn't exactly right. However, the lack of proper occlusion and obstruction support makes Battlefield 2 unplayable with EAX effects enabled.

We observed these problems on a Foxconn motherboard with an ALC882D codec, but any motherboard that uses a Realtek codec for High Definition Audio is likely to be affected. After all, Realtek handles 3D audio in the driver, which is shared among all of its HD Audio codecs. (Realtek's AC97 audio drivers may have similar problems, but we haven't tested those.) Fortunately, Analog Devices appears to be doing EAX right, but that doesn't help if you already have a board based on a Realtek codec.