Russ

Ok, there are some optimizations that you can do:

Use Outputs instead of temp registers when possible (save gprs).
Add the 2 channels together so that it is triggered if there is a signal from either channel.

Your code was not working for me, and I am not sure why, I will have to take a closer look at it.

In the mean time, here is the code I used while I was waiting (to show another (very similar) way of doing it)...

input in1_left, in1_right;
input in2_left, in2_right;
output out_left, out_right;

macs out_left, in2_left, 0, 0;
macs out_right, in2_right, 0, 0;
acc3 0, out_left, out_right, 0;
skip ccr, ccr, 0x100, 2;
macs out_left, in1_left, 0, 0;
macs out_right, in1_right, 0, 0;

To add volume controls, you would just change the first 2 and last 2 instructions to apply the volumes.

Ok, now let me figure out why your code is not working for me...