Does audio signal change polarity?
It switches back and forth. That's what "alternating" (the "A" in "AC") means.

The speaker cone both pushes and pulls from its neutral position.



However, in many applications, it is useful to add a DC signal to an AC signal so that the resulting sum is single-polarity. The most common example of this is the class-A amplifier. A DC "bias" signal is added to the low-level incoming AC audio signal so that the amplifying device is always in its "active" region. The amplified DC component at the output is removed (typically by a capacitor or coupling transformer), leaving you with just the amplified audio component.