Not IMHO kijanki, but absolute fact that at 7Mhz it will have no effect. At 1Mhz, worst case would be -170db down. I would be careful with the 1000 meter example. In that case, the worse case would be -60db error at 20KHz, which could be argued as audible. That is worst case though. Odds are it will be much better.
The idea that 7m speaker cable is a transmission line for audio signals
is insane, IMHO. For 20kHz signal you will need about 1000m long
speaker cable (1/10 of the 20kHz wavelength) to even start becoming
transmission line. In such case reflections would be inaudible because
they are in MHz range (and because speakers are 1000m away). Why not
to use 20kHz sinewave for the test? It is the highest audio frequency
component of interest in the cable. Please show me reflections of 20kHz
sinewave in 7m cable. Any cable.