Electronic dimming using triacs or thyristors use a technique which switches on the device at certain point after the AC power has crossed the zero reference line.
By nature of it's construction , the device turns off the passage of current thru it when the current is zero (i.e. ...at the zero crossing point. This is very convenient, since we can switch it on every cycle at a point very near the zero crossing or far from it. If we switch it on near the zero crosssing we get a brighter lamp since the device remains on longer before turning off t the next zero crossing.

This sudden turning on of the triac or thyristor creates transients which buzz very nicely on the audio line at 50 or 60 Hz. By putting large inductors on the output of the triac this sudden rise can be smoothened to a certain extent...but not perfectly. Since the largest current rise is at the centre of the AC waveform, you will find the largest amount of noise when the dimmers are at 50%.

Solution....several but none perfect !!

Use larger coils so that the rise time of the transients are slow.

Do not run microphone lines and dimmer to fixture power lines in parallel. Keep them as far apart as possible.

Sometimes a particular microphone/guitar will give trouble...change the cord or use a DI box with the Earth Lift swtich activated.

Last golden rule ...power the audio from a different AC source than the lights.

rgds

Ujjal Kar