It only costs $10 to file a letter of disclosure with the USPTO.
Before I spend a significant amount of money to file an application, I
want to ask a qualified individual to roughly estimate the
effectivness of an invention I came up with. My plan of action is to
file a letter with the USPTO, then have someone with a good background
in physics (especially acoustics) sign an NDA so I can share my idea
with them, so they can give me the their best estimate of how well the
invention would work.

Something I read on the USPTO website the causes me to suspect that
the disclosure letters are entirely useless is as follows:
"The two-year retention period is not a "grace period" during which
the inventor can wait to file his or her patent application without
possible loss of benefits. It must be recognized that, in establishing
priority of invention, an affidavit or testimony referring to a
Disclosure Document must usually also establish diligence in
completing the invention or in filing the patent application after the
filing of the Disclosure Document.

Inventors are also reminded that any public use or sale in the United
States or publication of the invention anywhere in the world more than
one year prior to the filing of a patent application on that invention
will prohibit the granting of a U. S. patent on it.  Foreign patent
laws in this regard may be much more restrictive than U.S. laws."

Questions:
1. a. If a company starts selling something similar to my product
before I get patent pending status, does that eliminate my ability to
get a patent? b. Would that eliminate my power to stop their
production when the patent is granted? c. Would such a letter to the
USPTO be a legal means to halt other companies from filing a patent
for the same idea?

From what I read at the USPTO website, it would seem that someone who
gets the idea can simply destroy any chance of a patent by selling a
single similar item, even after I've proven the idea was mine.

2. Exactly what is meant by "an affidavit or testimony referring to a
Disclosure Document must usually also establish diligence in
completing the invention or in filing the patent application"? What
exactly does the USPTO expect me to do?

I'm a layman in this issue, but I have a few thoughts after reading up
a little on the letter of disclosure. Perhaps they'll be of use.

About the letter of disclosure (quotes from
http://www.uspto.gov/web/offices/pac/disdo.html) -

First of all, you might be putting the cart before the horse.

"It is strongly recommended that the document contain a clear and
complete explanation of the manner and process of making and using the
invention in sufficient detail to enable a person having ordinary
knowledge in the field of the invention to make and use the invention.
"

Do you really have this 'manner and process of making and using' or is
this something that needs verification from the expert? Because if you
don't have this 'manner and process' then you only have an IDEA which
is not patentable in any case. I can't claim "the flying car" as my
INVENTION without concrete details of how the thing actually works.

This, it seems to me, is the point of the letter or disclosure and the
two year window. It is a way for me to say 'I have an invention, it
will work so and so and be of benefit in this way' and allow me time
to actually produce the item in a fully formed working state (hash out
all the minor details etc.). Thus, you can lay claim to a patent
without having to worry that, say, in one year's time IBM will start
working on the same topic and produce a working model before you do.
You will have shown by the letter that you were "first to invent"
which has priority in the eyes of the Patent Office (i.e., it's your
baby).

By the way, that's the "diligence" part you mentioned in your question
- you will prove that it's your invention by actually showing that
you're producing it. Contrast this will my flying car - it's just an
idea and I can't keep filing letters of disclosure every two years,
waiting for someone to actually produce one, and then say "Hey! that's
my invention! Pay me."

So, in my opinion, I'd first start with the acoustics expert and
figure out if you actually have a workable invention first (and one
that would be worth patenting - can you make money with it?).

Also, note that a letter of disclosure is useful but that "this
program does not diminish the value of the conventional, witnessed,
permanently bound, and page-numbered laboratory notebook or notarized
records as evidence of conception of an invention, but it should
provide a more credible form of evidence than that provided by the
mailing of a disclosure to oneself or another person by registered
mail."

In other words, there are additional ways to prove it was your
invention beyond the letter of disclosure (although for $ 10 I guess
it wouldn't hurt to file one).

I've relied on this website which illustrates the patent/selling a
product process:

http://www.willitsell.com/

You might want to take a look. As you'll see, the first step is really
figuring out if you have a workable, useful invention to sell. Then
you can worry about letters of disclosure and trying to get a patent.
That's basically what this comment was aiming to say (I must admit it
sounded much better in my head; it didn't quite translate).

I hope this helps. Have a good day.