I'm glad that you "get it" regarding the difference betweeen a DVD and an MP3 audio exeperience. I hope that Mazda does bring them back in the US.
The sound from the 5.1 is strikingly better! I played around with the settings the other day, when playing some 5.1 Talking Heads. I'm going to be digging out my other 5.1 discs (some Rolling Stones, the Who, and so on).
I tend to notice that even with CD, I get a fuller sound than with my MP3s. I'm not complaining about the MP3 sound, but as someone who appreciates the various different Beatles mixes - the mono over stereo on several, the DVD player really enhanced our drive today. Within Without You was swirling around in our car and my kid was trying to "catch" the sounds circling her head.
But yeah, someone will come back and try to convince me that CD/DVD is unnecessary and that convinence is more important and that you can't hear the difference blah blah blah.
These would be people who will argue to watch a movie at home rather than on an IMAX sound system for whatever reason, I guess.