MOC and JACK aren't playing nicely

I was playing around with JACK, and then tried to start MOC.

Can't load plugin libmusepack_decoder: file not found
Running the server...
Trying JACK...

FATAL_ERROR: Can't create buffer thread

FATAL_ERROR: Server exited

Other JACK-enabled programs I tried (Ardour, Audacity, Mplayer, Hydrogen) work. MOCP is from Arch Linux repos, ie compiled with JACK, so it's not that. Should I try and grep the debug files for more info or am I missing something really simple here? :)