Axle - Shielding is a very difficult thing. I assume you're talking about shielding speaker cables from external EMI radiation. I'm not sure that you need to shield speaker cables but in general shielding works a little different than we think. Shielding material of interconnect is non-magnetic but it is shielding cable not only from capacitively coupled but also electromagnetically coupled interference (common mode). EMI in fact IS electromagnetically coupled into the cable but because of skin effect it travels (to ground) on the outside of the cable's shield. Field inside of the shield is zero as long as cable is symmetrical/uniform.
When a cable shield is carrying differential mode current, as in the case of coax, skin effect will cause that differential mode current to flow on the inside of the shield.
When a cable shield is carrying differential mode current, as in the case of coax, skin effect will cause that differential mode current to flow on the inside of the shield.