my first thought on this would be to associate each sound to be played with a series of numbers.. and making a variable for that.. so imagine
PlayNote = 0 (this is your global variable)
every 1.0 seconds
set value - Play note - Random (1,12)
then you'd have
system - Play Note = 1 Play Note 1 sound
once while true
system - Play Note = 2 Play Note 2 sound
once while true
etc..
This would of course just create a bunch of random sounds.. but you may get the idea.. you could manually set the order of what notes you want..
On start of layout
Play note 1 sound
Wait 1.0 second
Play note 5 sound
Wait 0.5 second
Play note 3 sound..
etc..
Also you might want to check out the midi plugin someone made (you'll find it under tutorials, just look up "midi"), but bear in mind its only known to work on web. I was unsuccessful getting it to work on iOS, but i'm not too savvy with it and didn't write the plugin.