Whenever electricity (i.e: an audio signal) passes through a wire, it produces inductance that creates an electromagnetic field.
Because EMI manifests itself at a high frequency, it is commonly believed not to interfere with the audible range of frequencies. However, all frequencies demonstrate harmonic structures.
Even if the cause of EMI is outside the audible range, the upper and lower harmonics of a given frequency often present themselves within the audible range.
Digital equipment (although functioning at extremely high frequencies), unless electrically isolated, can/will affect the presentation of a sound system, in the same home, through the AC lines.
There are numerous threads on this very platform, regarding the problems some have had with things such as fluorescents and/or faders, making it past those beefy power supplies and causing audible interference in their systems.
EMI can be picked up outside of the system, via cabling, OR EVEN created by a signal, moving along the traces of a component’s printed circuits or internal wiring, not to mention: a nasty power transformer.
Of course: much of the effect of EMI, present in a system, may go unnoticed, until it’s gone and the listener experiences what some call, "a blacker background", or: "greater definition, clarity, etc".
So many variables!