Most musical instrument sound pick-ups, for example those in electric guitars, are comprised of high impedance circuits driven mainly by voltage, with very little electrical current flow. That is why handling noise (microphonics) can be a problem for guitar cables.
Microphonic noise is caused by the minute voltage generated when a cable is flexed, stepped on, etc. Guitar cables must be designed to prevent this, so a conductive PVC layer is placed under the shield conductor to drain away this voltage.
Note: This conductive layer must be stripped back when wiring, or a partial short will result.