Are the sounds on the 6 speakers supposed to be synchronized, like gradually each of 6 separate synchronized audio channels comes to life?
Are the sounds to be recorded sound clips, or generated tones or similar? Do they loop or just play once?
I think my approach would be to have separate MP3 players for each of the 6 channels.
Maybe something like this: http://arduino.cc/forum/index.php/topic,49979.0.html
Then the Arduino code has to command the player to rewind and play the clip on a given player. You'd have flexibility with different clips to choose from on each channel. But synchronizing the channels in any way would be tough.