thanks.
so make sure that the outer lug (the one marked sleeve on page 18) is the one wired to the right + pad like shown on the picture you posted. also check if you wired the ring contact of your input switch correctly.
however, based on your original post, it seems like this should be ok. if you switched the input's ring lug, the battery wouldn't work and there wouldn't be audio and if you switched the DC jack lugs, it should still work off the battery instead of the external DC supply. (sorry, I reread your OP and only realized this now). please check this nevertheless to make sure.
Pulled out the multimeter, one of the positive circuits reads 19V.
The other reads 0.
I unsoldered the circuit from the plug and tried it the outside circuit reads 19V, the inside one 0.
did you unplug the battery when you added the external supply? if so, only having voltage on the outside lug is normal (inserting the plug opens the normally-closed switch and disconects the inner lug from the outer one).
so, follow-up question: what's your power supply? and did you want to power it with 18V? if so, 19V is ok, but if you wanted 9V, 19V is definetly beyond the usual fluctuation. according to the build doc 18V should be fine anyways. And just to be completely sure, you are powering it with DC voltage, right?