what those things even do?
There are several types of approaches... more serious, more idiotic ones.
The best one is the principle a online-USV uses, create a complete new AC. Depending on the quality of the unit this can be more or less noisy.
Other products do other things, like loading with resistors to "symetrize" (or do whatever) the AC. Reshape the AC by adding the chopped caps of the sine-wave (normally they are chopped of by the rectifiers of SPSUs), create blue light...
Anyway, if you really need such a Unit for Home-Audio, you either life (electrically wise) in a 3rd world country and will have much more serious problems or... you don't need it