Why won't my Vibrant play any system or media sound?

I recently purchased a Samsung Galaxy S (Vibrant) T-959 off of ebay and it has the most unusual audio problems. It will not play any media sound or system sound through the external speaker. The handsets' built in earpiece will not transmit any sound either. And lastly, the headphone jack will only transmit audio through a single earphone. At first, I figured this would be an easy fix through the cheap and simple replacement of the earpiece speaker/ headphone jack / light sensor assembly that comes all in one piece. This made no difference once installed and the malfunctions continued.

What makes these audio problems even more perplexing is the fact that the external speaker does in fact work. The phone will play ringtones etc, but nothing else. I then took the cellphone to a repair shop where the service people were convinced it had to be a software issue. They upgraded the operating system to Android 2.2, but once again the problems persist. I am now at a complete loss. Any ideas?

P.S.

Yes, I did try adjusting the volume levels under system settings...