How are multiple frequencies played through a speaker/buzzer without distorting eachother?
This is really a lengthy comment as apposed to a answer to the question.

What you are trying to do is a bad idea if the point is the result.  If you're doing it for fun, the challenge, or learning, that's fine, but don't expect good results.

It's going to be very difficult and tedious to keep all those analog oscillators from the 1970s in tune.  Ditch the evil 666 555 timers and do it all digitally.  Not only will the result be better, but it will be much easier too.

Add all the signals you want to combine digitally, then write the sum to a D/A and have that eventually drive a loudspeaker.  Write a fast lookup-based sine routine, then use it for each of the tones.  The frequencies are controlled by how much the indexes into the table are advanced each sample.  Use maybe 16 fraction bits below each index to get plenty of frequency resolution.

Make the sine table a power of 2 in size.  That means the indexes (angle arguments to the sine functions) automatically wrap to 0 when incremented past 360&deg;.

Even brute force 10-bit sine lookup will yield far better results than a bunch of 666 timers.

This will be easier on a processor more meant for such things than whatever comes in a arduino.  Something like a Microchip dsPIC can easily do this.

Let's see how the numbers work out.  A dsPIC is inherently a 16 bit machine, so let's use two words for each angle.  The high word will be the table index directly, and the low word fraction bits for more resolution.  A EP series dsPIC can run at 70 MIPs.  Let's say you want 40 kHz sample rate.  That means you get 1750 instructions per sample!

For each contributing tone, you grab the high word of the angle, use it to look up the sine value from a table, add that into the accumulator, then bump the index by adding its 32 bit increment to it.  let's say this takes 25 instructions per tone, including all the initialization, loop, and termination checking overhead.  It should take less than that, but even with that this method can support 70 tones.

That's way better and easier than trying to keep 70 analog oscillators tuned.