This is looking like yet another case of Samsungs lack of production consistency for these phones. There's a few other topics here about this very issue, and I've seen it in other forums as well.
Also in the list: GPS, Battery, CPU/power control circuitry, screen, and now speaker.
Since physical contact/movement of the speaker seems to get it working again, I'ed venture to say your options are:
A. Crack open the phone and check the speaker or connection circuitry. If your good with microelectronics, you can shop for a "parts" phone and replace it.
B. Call up and complain/talk them into having it replaced/repaired.
From my knowledge of speakers and small devices, it appears that the speaker was not installed properly in the phone and can move into a position where there is no power to it, and thus it does not work. OR The physical construction of the speaker is faulty and some internal part separates causing power to not be sent were it needs to go, and thus it does not work.http://www.techrepublic.com/photos/samsung-Exhibit-at-t-samsung-galaxy-s-teardown/456848
We could relate this phone to a "Russian Roulette" game, in that either your lucky and you got a "perfect" phone, or well... BANG!!