As long as you use a micro-controller with two PWM hardware modules it should be no problem, all you need to do is read each byte from the MMC card and write it to the PWM modules. I would suggest that 8 bit resolution is plenty for an elevator, and you probably don't really need stereo either?.
Sorry, I usually don't chime into things like this but....
It seems like a MUCH better idea would be to fashion a pre-made device or circuit for this. Such as a small cheap MP3 player with your micro integrated in.
If you try and re-invent the wheel you may have trouble getting it to be as round as it could be.
Well playing a wav file from a memory card is the easiest. I done this exact thing. Using a dsPIC30F4011 a SD card and PWM for output. It could play up to 48Khz 8Bit Stereo. And i must say the sound quality for the PWM can be better than a cheap MP3.
One problem with wav files is that they are big. You might use AC3 as suggested , microchip made some code for that. As for MP3 and such it cannot be decoded right on a pic because its too slow.