Do materials alter frequencies and speed?

Frequencies and speed is not accurate to describe the effect. Geometries aside (spacings and wire gauge etc) , materials in cables have several effects, both insulation and metals. Lets start with metals:

The crystal lattice in the metal can cause reflections in the cable, because they are impedance discontinuities. These occur at very high frequencies, but this does not mean that they will not become audible. Depends on whether the driver reacts to these reflections or not. most of the time, this is audible and sounds like sibilance on top of the high-frequency information in the music. Metal purity, choice of metals, cryo treatment and other factors affect this.

The insulation has several effects as well. Besides the obvious effect on capacitance per unit length, the dielectric material absorbs charge and releases it when transients occur. Depending on the material, it is not completely released either. In general, if the time-constant of the material release is slower than the time-constant of the transient, smearing will occur. Dielectrics with a lot of air will usually have the least dielectric absorption effect. The effect of this is usually a loss of HF dynamics, often described as a roll-off, but technically it isn't.

Rsbeck - you can see plots of measured and computer simulated response of ZIP cord versus my speaker cables at this website:
http://www.empiricalaudio.com/frclarity7_electrical_performance.html

The simulated and measured are both obviously rolling-off with ZIP and have different phase response. The only issue is whether this is audible or not, and based on my listening tests, it is. My cables must be doing something right as they were favorably reviewed in April Sterophile, and I might add that I have never put any ads in Stereophile.

Steve N.
Empirical Audio
Manufacturer

Listening tests - I do this at every CES. I have some 11 AWG ZIP and I swap it with my speaker cables. I do the same with IC's and digital cables. The listeners hear the difference every time.

Steve N.
Empirical Audio
Manufacturer