I have found ERS most useful on treating AC, and placing it on top of or underneath digital gear. I have had mixed results treating interconnects with ERS. I haven't done any speaker cables yet.
I have had excellent success treating power cords, and power conditioners.
The only downsides are, ERS ius conductive, and you have to be careful working near any exposed conductors, and the tape is not the easiest stuff to work with.
I recommend doing a "trial" run of wrapping a cable before taking off the wax paper and exposing the adhesive backing of the tape.
Allan at Stillpoints recommended treating AC as close to the source as possible. He recommended treating the wire coming into the house from the transformer, and the circuit breaker box. I did this to the 220v wire running to my house. I had easy access to it since it pops up through a pvc tube next to the power pole. I treated about 4 feet of it and may do more. I also treated my breaker box. I really, really liked what it did.
If you attempt this, BE CAREFUL!!! Better yet, have an electrician do it for you.