No luck.
EC PlaySound was not set to run as administrator, so I updated that. Restarted everything, logged into Test Center 1, clicked the test link, and nothing happened. I checked my processes, and EC PlaySound was indeed running (and it works - I tried changing my music settings to the old music then back to the new EC PlaySound music, and that worked fine).
I exited UO and set the game folder C:\Program Files (x86)\Electronic Arts\Ultima Online Enhanced to Everyone: full control and confirmed that subfolders inherited those permissions. They did. I restarted the game, logged in, hit the link in my browser, and nothing.
I then tried my own link on my home shard (Atlantic) to see if I could get it to locate my vendor. I tried:
pincoui: -locate -x 782 -y 1151 -facet 4 -shard atlantic -name Battle Chicken Goods
Nothing happened. Double-checked that EC PlaySound was still running - it was. I double-checked that EC PlaySound had everyone set to full control. It did. I tried the link above without the -name option (in case the spaces in the vendor name were causing issues). No luck.
For fun, I tried setting EC PlaySound to XP SP3 compatibility mode. Restarted UO. No luck. (I'm running Windows 8 64 bit. EC PlaySound shows up as a 32 bit process).
Rebooted the entire computer, just in case one of the changes didn't take. No luck.
I checked in the browser to make sure that the pincoui URI was set to open in EC PlaySound. It is.
I'm running out of ideas!