Typical loudspeaker cables have a constant impedance in the audio frequency band so the voltage drop in proportional to signal output and there is thus no frequency response change. Even if it wouldn't be constant it would just cause a linear distortion (frequency response change) but no intermodulation, as for this a non-linear behaviour is necessary and such doesn't exist in the cable, at least in the low frequency range we are talking about.
To easier show that a signal in a linear ohmic cable won't modulate another signal lets take the above mentioned example with loudspeaker impedance Z2=4 Ohm and cable impedance Z1=0.1 Ohm which are connected in series to the amplified with output voltage Vin and thus create a simple voltage divider:
The voltage difference Vout which is applied to the loudspeaker Z2 is always proportional to the input voltage (which is the output voltage of the ampifier), namely according to the equation:
so in this case 4/(0.1+4)=0.976 thus 97,6 % of the input voltage from the amplifier.
Now lets assume a high frequency sine with an amplitude of +-1 V, at the loudspeaker the maximum applied voltage of the sine will be 1 V * 0.976 = 0,976 V and the minimum one -1 V * 0.976 = -0,976 V and thus the total amplitude of the sine will be 0,976 V - (-0,976 V) = 1,952 V.
Now lets add to it a low frequency oscillation which at the point of observation has an amp output value of 10V, the high frequency since will be added to it making the output of the amp oscillate between 10 V +1 V = 11 V and 10 V - 1 V = 9 V. The loudspeaker will see at those points maximum 11 V * 0.976 = 10,736 V and minimum 9 V * 0.976 = 8,784 V and thus the amplitude of the high frequency sine will be 10,736 V - (-8,784 V) = 1,952 V and thus the same as above, the beauty of LTI systems.