Right, you probably said that in one of the first posts in this thread...

If the signal is going in to channels 1&2, then it should be sent internally to channels 3&4, or so the manual says, so make sure you're using channels 1&2. If still no sound, try to adjust the gain(level) on channels 3&4. If you still don't get any sound, I'd say it looks like the amp is broken...

Stig