I'm trying to wire my e-fan and have some 10 gauge audio wire laying around. my question is, is it ok to use audio wire instead of regular wire.

 

The insulation on the speaker wire might not hold up for long at the high temperatures experienced in the engine compartment. Speaker wire typically uses PVC insulation, which is good for maximum temperature of about 100C.

 

A 10g wire should be enough, it really depends on the needed length and the max amp pulled from fan, but given the length of the engine bay, and the fact that most quality fans pull about 30amps at peak, you should be more than good. Here's a chart that shows gauge, length, and amperage:

https://www.crutchfield.com/S-oAcWuw...uge_chart.html