Metal's conductivity is dependent on temperature, but not enough so for such small variations as a 100 degree range. We had a large (and largely sarcastic) debate a year or two ago about condctivity of different materials, and I had some old reference books at my desk at work with conductivity values that I referred to at the time - they probably gave some guidance on conductivity in relation to temperature, if you are interested.

Power wiring can be warmed by the current, but if the wire is sized per code the heat generation should not be significant (especially at the 120V level used in homes). This is actually why you often hear warnings about aluminum wiring in homes - its lower conductance causes it to heat up more, making it easier to overload a circuit enough to create a fire hazard if you aren't careful. Speaker wire should experience less heat gain from current than power wiring, especially if you use a good gauge of wire (14ga or 12ga should handle most any typical home theater speaker load you're likely to have, doubly so for surrounds).

You mentioned condensation, but I will behave myself and leave the psychrometric chart in the drawer. Let's just say that heating a surface will push you away from the dewpoint and make condensation less likely. We worry about condensation on cold surfaces - hot surfaces are safe.