First game I cared to get through from start to finish with EAX - Call of Duty 1 and UO. I actually played both games through United Offensive expansion's executable.
Used Singleplayer Improved 1.1 mod and CoDUO-YAP. The author of the latter fixed both FOV and HUD for widescreens. Cinematics work too when wide resolution is used, preserving original aspect ratio, menus also use 4:3 ratio. Now if only mouse buttons 4 and 5 were natively bindable.
On the sound front, it does seem to be the clearest through Creative's OpenAL driver. Very distinct from software mode in ALchemy's dsound.dll. Or using Sens_oal.dll renamed to ct_oal.dll, that seems to fail to produce EAX effects. With both of these, it's apparent that weapon switching sound sounds off as well. Muffled and somewhat distant.
I'd say OpenAL Soft ranks second, but not quite there in clarity. It's possible my config could use improvements, but then you need the know-how. And effects are somewhat stronger than on Audigy Rx's OpenAL. I'd say it's more balanced on the latter. No idea what other bugs (if any) could be there with ALchemy and certain other games, but at least for CoD, in terms of sound, I can't say it sounds any different on XP, where DirectSound is fully featured, compared to newer OS with ALchemy.
CoD's sound engine is restricted to max 32 concurrent sounds. Hectic sequences do suffer from this limitation, there are audible cut offs. But, it's not as simple to increase the limit as one might think. It does query MSS library for max sound channels, that seems to return 95 on my system (which is over Audigy's limit) if I recall correctly from recent poking with debugger, this is capped by the engine to 32. And naively increasing the variable with the limit crashes the engine.
I also compared EAX 3 mode in one part of the game with 2 and 1. Sewer level, after last autosave, the corridor leads to outside area with buildings. Being inside the corridor makes the nice suppression effect on the shooting sounds from outside. Though with EAX 2 and 1, no difference from being in the corridor vs. outside. I guess one of the standards presets is used, shooting in there will also reverb.
One last thing, the game apparently doesn't save EAX state, so if you saved while being in such area, it won't be restored on load, only passing the trigger again in the world will activate it.