The led for HMUTE is driven by IC3 from pin 7 through R11, that's the one not working right? IC3 is a simple Cmos kind of chip that can turn the output ports high or low with sufficient current to drive a led directly, and so it does.
I am not entirely certain what input pins it gets triggered by, could be a couple of them, could be that the output is pulled down for some reason other than the logic output of the TA4100A not working or logic inputs or output on IC3 not working. It could be as simple as a little short from a little solder blob that pulls it down...
Only way to tell for certain is to measure it all the way through, resistance to ground and other pins, voltage on the input pins etc. etc. And it couldn't hurt to check the datasheet of IC3 for the whole pin-out and logic operations.
This should help:
Last edited by V-bro; 11-May-12 at 00:01.
Pics can help a lot to diagnose something from any place on the globe...