I am not sure which phase of the checklist you are in. My operator would switch on the CVR as part of the Overhead panel scan. The Beacon and doors checks would fall into our Push/Start checks.
There is an important distinction, because (in our SOPs) whilst the set up is going on the Pilot Flying would be doing the walkround. The set up of the panels is normally a sole job, (with PF checking when he/she returns to the flight deck) so it is a memory drill, and is conducted in silence.
The Push/Start checks are called for by the PF after a clerance has been obtained. The PNF will do the switch selections and scans (For us, its Belts, Beacon, Windows and Doors, Thrust Levers, Transponder, Cockpit Door. Nose Wheel Steering) and will then pull the checklist, and challenge/response with the PF.
Skin Air Heat exchange system is a method of supplementing avionics cooling, as EI Mick has stated. When airborne, the skin air valves close, and air is circulated through the avionics bay, and around the space between the aircraft pressure hull, and the outer skin. This makes use of the low skin temperature aloft to assist cooling. It uses a blower and extractor fan to force air round the system. In the event that smoke is detected in the avionics bay, then the blower fan shuts down (in case it is the reason for the smoke!) and the extractor continues to run, and a dump valve on the skin opens topurge the smoke overboard.
The air conditioning packs consist of two elements - a heat exchanger system, and an air cycle machine. At high altitudes, the packs draw outside air in, which is passed over the heat exchangers to cool the air in the system. This conditioned air is then delivered to the mixing manifold and thence to the trim valve system. However, at lower altitudes, or in hot conditions, there may well be insufficient cooling from the heat exchangers, and the air cycle machine will run, which is similiar in principle to a domestic fridge compressor.
I hope that helps...
Cheers