"// an array
[ 0, 2, 4, 7, 9, 11 ] @=> int hi[];

while( true )
{
    Std.mtof( 45 + Math.random2(0,3) * 12 +
        hi[Math.random2(0,hi.cap()-1)] ) => s.freq;" ?

Yeah, Math.random2() is one of the built-in functions and "you-name-it.cap" 
would be the number of elements in "you-name-it[]" 

so you'd be picking a random element of the array "hi", getting the frequency of that as a midi note,
putting it into the oscillator ("s") you started a few lines back.