Actually, since audio signals are AC waveforms, speakers use AC to reproduce them. DC can only move a speaker driver in one direction from their neutral point -- outward (or inward if wired that way). Note how all the energy in the red dc line lies above the horizontal axis:



If you send a mixed AC and DC signal to a speaker, the DC component will effectively move the speaker's neutral point outward (or inward depending on how the speaker is wired) and increase the likelihood of distortion.