Re: Min distance for low volt cables
What Wayne is saying is that the speaker cable, or any other low voltage cable for that matter, has to be listed or it cannot be run concealed in walls, ceilings, etc. This means no "zip" cord such as many speaker cables are. Make sure your cables are marked CL2, CL3, CM, CMR, CMX etc.
As for spacing, there is no requirement however I would try to keep speaker wiring away from noise sources such as the NM running to and from dimmers, etc. Probably won't matter but it can't hurt.
-Hal