> Is the square wave that comes out of a 555 something that can be
> used, then filtered to make different sounds?

Absolutely. Even varying the duty cycle during a note can sound
interesting.

You can also get a triangle wave from a 555, which is less harsh
on the ears than a square wave. A few components can convert
the triangle to a sine wave. The Control Voltage pin (5) can be used
as a modulating input, for example to make the 555 the basis of a
voltage-controlled filter. 4016/4066 switches are put in series with
resistors in a typical audio filter and the 555's duty cycle is altered
(via CV) to change the tone. Which could be anything from a simple
bass-treble control to a sweeping wah sound. The audio source
can be another 555

With a handful of 555s you could make a small basic old school
modular synth - VCO, VCF, VCA, LFO etc

I've seen a circuit which used the 555 as (I think) a pulse-position
modulator as an audio amplifier. I remember the author said it
wasn't fantastically good but demonstrated a principle

Have a Google for 555 Cookbook, plenty of odd circuits around

Joe



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