A good question. Do those cables have any circuitry in them? Just going RCA to XLR will not produce good audio, as the XLR side is balanced, and the RCA isn't - so you'll get impedence problems. You need a balancing/unbalancing amp, transformers, or a small mixing desk to solve your problem.

You may find more info by searching, but here's an example.

Here is a link that might be useful: Wiring diagrams