(1) Re-try the customized Hijack for us. It won't work, but we gotta know!!

Check the fuses. When the hard drive cable is misplugged, or the display cable misplugged (forgot which), sometime it blows fuses near the display cable header. But usually music still plays, just the display ends up buggered. So that's not likely the case here. The fuses are the three identical looking two-pin items mounted around the display header; two on the rear left (nearer the docking connector), and one on the front left. Measure them with an ohm meter, and they should all read zero ohms.

But still looks like a bad sound chip, or maybe just mis-soldered. Check the +5v and GND pins on the sound chip that was replaced.

Cheers