add a temp gauge or "dimmer" to cabinet fan
They make much quieter fans for this. There are many how-to guides out there on how to do entertainment center exhaust fan systems. You can google Entertainment center exhaust fan and see some pretty slick designs. There were a couple that were 'automated'   

If you are using 120v you could always get a Attic Fan Thermostat and connect it inline with the Fan Power. something like this unit. It is settable from 50 degrees to 120 Degrees. Or something like this if you want it to look nice outside of the cabinet.