Without knowing your setup, it's going to be difficult to exactly troubleshoot this.
I would guess that the problem is with the external setup rather than with the iPhone though. I know it's teaching grandma to suck eggs but have you made sure that every cable is plugged in to the correct input/output and everything is powered correctly? If not, you could 'bread board' it, to use a phrase from PC building, by laying everything out as best you can and following leads from start to finish to check.
Occam's Razor is usually the answer here, from my personal experience.