I'm not going to dispute what you're saying, but an audio signal _is_ AC (alternating current). It is referenced to the circuit ground, but is not correct teminology to say that it is DC (direct current). If you say that an audio signal is DC around an engineer, he (or she, or it) will respond with 'huh?'
It is not DC since it is constantly changing, DC is defined as a static voltage that doesn't change (at least much).

I know, I know, that arguments can be made to the contrary, but these are the generally accepted definations of AC and DC, except in some parts of West Los Angeles.