:dubious:
The two definitions I left in seem to agree with Napier’s (well, IEEE’s) definition, not yours. The rest just seem to be whatever definitions Google found on various websites. Hardly official.
The signals are a DC level. Old vehicles used generators, which produced DC current, but this was not nearly as efficient as the now-used alternators. Alternators produce AC which is rectified (converted) to DC. Once it leaves the rectifier (usually built-in to the alternator), you won’t find AC in the vehicle. By the way, the reason AC is more dangerous is that it is about 60 cycles (per second). This is close enough to the heart rate to stun it. 'Nuff said…
No, it’s not, unless you’re a hummingbird.
Trying to go straight to the SPIRIT of the question, based on the kind of simple questions a non-expert like us would ask.
I think he is wondering “wouldn’t alternating the polarity would affect the quality of sound?” (like resonating any frequency multiple of 60Hz).
I was wondering too, and my answer is: I doubt any of the 60Hz alternation is kept on purpose on an audio output.
Why?
Something most audio signals share is they are designed both for us to hear and for a speaker.
Speakers cones bounce, and NEED to go back to their original position, in the middle, like any (most?) sound generators, even our vocal cords.
They try to be efficient and having the cone always moving from the middle forward, or only backwards, would be not only not-efficient, but it’d also spend too much energy on keeping the cone always pulled to that one side.
Now, more or less precise, the bouncing speed of a speaker must be thousands of times slower than the lower audible frequencies, so for the speaker cone go back to its position, it will be rather PULLED by the coil, than going back by itself, so the polarity NEEDS to be reversed.
So, I don’t see how the frequency required to produce sounds with a speaker could PULL the cone back to complete the the wave, without reversing the polarity.
I guess compensating for the exact type of speaker used (not only with those nerdy electronic values, but rather mechanically) is the key for perfect sound 
I also thought about building a zapper with a regular sound card, (I have a small Raspberry Pi mini-computer that I expected to use to control it) but I concluded that the type of sound card that would work for zapping all I want, would cost more than a frequency generator so I bought one used off ebay, and now I just have to laugh about the silly probability of some thunderstorm or defect in the device make more power pass through the cables and kill me.
My final conclusion (for what I think is the relevant info here) is that none of the AC from the grid should be affecting the output of a sound card or frequency generator, despite there’s some polarity alternation in the output or not.
Excuse my lack of electronic lingo and basic english
For what it’s worth, this thread is five years old, your post just having resurrected it. Not that that’s necessarily a problem, but you may wish to be aware of it.