In my house, there's a gas fireplace that is housed in a 5'(W)x3'(D) wall enclosure. Poking into the enclosure, there's nothing around it. Just empty spaces.

Now on the opposite side of that enclosure is the garage where the main circuit panel are found. I want to run some several wires through this enclosure as it serves a convenient routes.

1. If the cable is only of low voltage, i.e. audio and 12-volt dc line, do the cable has to be run in a conduit or can it simply just sit in that open space inside the enclosure?
2. From the garage and the main circuit, I want to run additional 12 AWG wires, will probably be plenum rated. Do I still need a conduit as well?

I suggest you check the requirements of clearances for the fireplace. The space built behind these appliances may be dedicated that no combustibles be close to hot surfaces of the fireplace. If the cables can be kept away from any heat, out of the clearances needed,then running cables in this space should be fine.

Having wiring in conduit does not eliminate the potential of heat damaging the wiring. Plenum rated cable is not heat or fire resistant. All this type of insulation does is when it burns or melts it does not give off toxic fumes.

I suspect there is enough room to run these cables and support them outside of the distance needed from the hot surfaces, but only you can determine that by reviewing the specs for the appliance.