Alright, I just did one of these the other day. There's two ways to do it. Is the light, the led On on your sound board? I messed with mine for a long time, it wasn't getting power to that board so the light didn't come on. If the light is on, you're ready to go. Also, there's a sound test in the test menu that will test the sound board. If the board is bad, the 'checksum' won't come back and it will give you an error.
If you find the manual online, the soundboard sends sound out of the connector towards the bottom of the board (away from the jamma connector). The original cabinet connected there... that's how I wired mine up. 3 speakers, two wires each.
I BELIEVE, you can jumper the wires from the left speaker output on that connector, down to the connector on the edge of the board near the jamma connector. This basically routes the sound back into the main board, and out the jamma connector to the speakers hooked up to the jamma connector. I haven't hooked it up that way, but I think that's how it's done.