Here's a tweak I made that I think keeps to the spirit.

import random, time, winsound

timebias = 0.2
pitchbias = 0.7
changebias = 0.75

current = [(4.,1.),(5.,1.),(7.,3.),(None,1.), (4.,1.),(5.,1.),(7.,3.),(None,1.),(4.,1.),(7.,1.),(12.,2.),(11.,2.),(9.,2.),(9.,2.),(7.,1.),(None,1.),(2.,1.),(4.,1.),(5.,3.),(None,1.),(2.,1.),(4.,1.),(5.,3.),(None,1.),(2.,1.),(5.,1.),(11.,1.),(9.,1.),(7.,2.),(11.,2.),(12.,4.)]

timeshift = 1;
while 1:
    timeshift = timeshift + timeshift * random.uniform(1 - timebias, 1 + timebias)
    if timeshift > 1.0 + 2.0 * timebias or timeshift < 1.0 - 2.0 * timebias:
        timeshift = random.uniform(1.0 - timebias / 2.0, 1.0 + timebias / 2.0)
    key = random.randrange(0, len(current) - 1)
    if random.random() > changebias:
        if current[key][0] is not None:
            current[key] = (current[key][0] + current[key][0] * random.uniform(-1.0 * pitchbias, pitchbias), current[key][1])
    else:
        current[key] = (current[key][0], current[key][1] + current[key][1] * random.uniform( -1.0 * timebias, timebias))
    time.sleep(random.random())
    for (p,d) in current:
        if p is None: time.sleep(0.2*d * timeshift)
        else: winsound.Beep(int(440*2**(p/12.)), int(200*d*timeshift))

Basically, each loop it tweaks the song slightly from the one before it, randomly. The three different bias settings on the top dictate how the song evolves. But besides just changing the song, the rate of any play varies randomly (according to the timebias as well).

The timebias applies to changes of timing. So the tempo of the play, the rate of change of the length of a note and the length of pauses are all shifted by the timebias randomly. increasing this number will create more dramatic swings in time changes from run to run (as well as the overall bounds of the tempo).

The pitchbias applies to pitch changes. Increasing it will let the algorithm drift from the normal song much faster. Too high will cause obvious swings in notes. Too low, and it'll take forever to get a decently maddening change (but perhaps that's part of the master plan).

The changebias indicates the chance that on a particular loop, the pitch of a random note will change, or if the duration will change. This change is carried on to all future plays (and will have a ripple effect)

The result is quite maddening, as parts of the song will randomly trend back towards the correct notes. And notes you could have sworn were wrong will appear normal later. And back and forth it goes. Just repeating, and changing until you get driven mad (or bored) enough to ^C...

Basically, it's a genetic algorithm without a binding fitness function. Its random changes will just propegate infinitely towards chaos. But for a very long time it will have the "feel" of the original song...