May be non-linear phase response or cable reflections/electrical resonance. This can be caused by cables, particularly long ones. Lossy cables can cause general musical soup as well, particularly in the high-frequencies. This effect was obvious in my system when I immersed some IC's in LNO2 as an experiment. It damaged the surface finish of the wire, I believe. Sounded like the music was coming through a tunnel. The correct way to do this is to lower the temp and raise the temp slowly. It was just an experiment, but yielded useful information.
You can learn more about electrical resonance at he Audio FAQ page of:
http://www.empiricalaudio.com
including the results of the cryo immersion experiment.