Let us clarify:
1) Light switches triggering, this means it is the spark energy when switching. Early radio used “spark gap” transmitters to transmit Morse wireless, the same physics are at work here.
2) The problem is that the spark impulses use the house wiring as antennae transmitting the signal.
3) On the Audio side all and any cables become an antennae receiving this “blip” signal. If a wire is shielded and the shield is grounded/earthed, the “blip” should be rejected.
The probable cause here is either poor shielding or a lack of grounding or both.
It is unlikely that adding any form of isolation will help, as this will not help the shielding/grounding.
It may be “unsolvable” if the house has a poor earth connection (we did experience this in one apartment our colleague had).
The best solution is to defeat the actual spark. This may be done by having a competent electrician replace the mechanical relays/switches with modern electronic alternatives, or by applying suitable “snubber” capacitors across the switch to solve the problem at the source.
However, it is possible we are wrong but its a good starting point.
