You know I haven't seen any difference too. And yeah the Midas sounds better then the D sound, but Pete is working on the D sound. But did you try out the ePSXe 1.4.0 internal sound. It's the best so far. Well it works great for me, but check it out.
Try Iori's v1.46. It's better than the internal core because its feature set is more complete (The internal core STILL can't do reverb).
As to the question, MIDAS was a sound driver system developed before DirectSound came to prominence. It relied on standard sound calls, which meant nearly any sound card could use it. With the increase in DirectSound-enabled cards that provide more direct access to a card's internal features, potentially significant speed and performance boosts are possible over traditional sound engines by coding for DirectSound.
The hitch is that it's more difficult to code. That's why there are some hiccups right now. These will eventually be addressed, and DirectSound will likely become the system of choice for most users (such as myself, an SB Live! user).