What's the best cable to transfer audio signal?
There are two reasons why speaker cables are not shielded/screened:


The signal is so powerful that any interference would not be noticed.
Speakers are not very sensitive; it takes a lot of power to create sound on a speaker


This is why speakers are connected to an amplifier.

Input to amplifiers are very sensitive and so input should use shielded/screened cables.

P.S. If you have a 100W amp the good ones use about 48V. So you have at least 2A, with peaks higher. Peaks might hit 20A but for a very short time. To hear this your speaker cable needs to be thick.