Re: Why does the pitch rise as solids are stirred and disolved into liquid?

To understand why the pitch of the sound varies you must first
understand what exactly sound is and how is it produced.

So, what is sound?
In the Collins English Dictionary one can find this definition:
"anything that can be heard". But what does our ear do? Like every
sense organ it transforms some kind of energy into electrical energy
of the nervous influx which reaches the brain. But we don't discuss
here biology. The question is what kind of energy does the ear
transform? It is mechanical energy; because sound consists of some
ordered movement of (air) particles. Surely you have felt this when
listening to music at high volume.
In physics a 'simple' sound is a mechanical wave at one frequecy,
roughly between 50Hz and 20KHz. As opposed to a more 'comples' sound
which have a spectrum of frequencies, that is, it is a combination of
some 'simple' sounds. For the rest of this answer by "sound" I mean
"'simple' sound".

How is it produced?
In matter (as opposed to void) every local mechanical perturbation such
as a vibration will propagate. This is a mechanical wave. So anything
which vibrates is a source of sound, as long as it vibrates with the
right frequency.
A very important point is that every elastic body (and all bodies are
a combination of elastic and plastic) have a NATURAL frequency, which
depends only upon its internal properties. For exemple in the case
of the mass-spring ideal elastic body the expression of this natural
frequency is:

niu = sqrt(k/m) / (2*pi)
where niu is the frequency, k is the elastic constant (force per
displacement) of the spring and m is the mass.
You can find details about this in any textbook about oscillations
(e.g. A.P. French's "Vibrations and Waves")
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You can see from this equation that if an elastic body opposes more 
to deformation (big k) it has a bigger natural frequency, that is a
higher pitch.
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When does an elastic body oscillates?
To answer this question yourself you must think to the simplest exemple
of elastic body, that is the spring-mass system.

So, why does hitting two solids (rigid) produces a sound?
Because hitting means deformation, and deformation means vibration.

If you would like more details please don't hesitate to write me:
rgrig@geocities.com