If I want more length to the coils, can I just use regular audio mini plug female - male extension audio cable? Like 1/8" 3 pole
Would not do that.
Increases the resistance + additional contact burden in the coupling (resistance) + inductive issues. Will screw up or “flatten” the 100µs impulses and degrade the performance to a certain degree.
This is the reason I for example own a P9 in addition to an M1.
The micro pulse site has them. I assume these likely are higher gauge cables than your Amazon cables
Also micro pulse sells coils with cable as long as 47" it seems.
Maybe il still hold off until I see more feedback
Better minimal lCES than no ICES 
But I would try not to use the extension and only the pre-manufactures coils with long cables.