You could also thicken this up with a CD4040B 12 stage binary ripple-carry counter. 12 stage is kinda overkill, but whatever.
What you do is setup one 555 as an astable circuit, then feed the output (pin 3) of the 555 into pin 10 of your ripple counter. Then, the buffered outputs (Q1-Q12) will then give you 12 octaves worth of frequencies.
Combine this with a op-amp mixer, and you got a mighty chungus of a voice. What might be more musical though, is having a 1 pole, 12 position rotary switch to select which buffered output to use, effectively selecting which octave you want the voice to be.