Are those things possible
Not directly, it is the processing power of the normal Uno that you are going to have trouble with. Also writing a program that accesses and feeds different SD cards and D/As in an apparent multitasking form is not so easy either.
What I also like to do is split the stereo channel from 1 output to 2 different mono streams.
Wave shields are mono only.
You could use MP3 modules and just have the Arduino push the buttons.
Shows two audio files being read at the same time but the quality is very poor due to the method of outputting the sound using PWM.