Yeah, well, some speaker cable IS shielded and it's usually for fixed voltage ysstems- I have seen and installed it when I worked for an audio contractor in the late-'90s. It's used for installations where the areas where the runs are crowded with many types of cables and with different signals- some buildings need cabling but access by human installers is difficult, if not impossible, so it's a matter of "get it in and we'll let shielding be our friend. If a problem occurs, we'll deal with at one end or the other, or both".
Belden makes 12/2 shielded cable that's made for this- I'm going to contact them to get more info.