Given the extremely high speed of propagation velocity the delay factor would be as you note insignificant (due to the lengths of audio cable). But no doubt that insulation/dielectrics exert some level of sonic influence.
For example the maker of Ocellia cables is adamant that there are characteristics that favor natural materials (cotton, paper or silk) vs synthetic/plastics in regard to dielectrics. Obviously I’m in no position to affirm or reject this idea other than to say that so many variables affect what we hear. Granted some variables are more recognized and supportable than others. Audio is fascinating, television/visual domain doesn’t seem as mysterious or interesting (or maybe I'm simply bias toward this field). There’s just something about listening to music and its effect on humans.
Charles