LG very clearly stated in its marketing material that it has two microphones, and it uses the speaker as a 3rd microphone in high volume environments in order to capture a non distorted sound with better base capture. I am 100% sure about reading this on LG website at the time. I also had used it in such environments and the phone is one of the best sound recorders in the whole phone market.
Its possible that they used special components for the speaker. All of the LG V series' audio components were special. That was why people bough them.
I do understand a little bit about the electronics of this myself, but not enough. The last stage of the audio component is the amplifier. Various kinds of signal return dampening is used in the amplification stage (that I don't understand that well either) to control the signal "feedback" that gets pushed back into the amplifier from the speaker. So how it all works exactly I don't know for sure. But I'm sure an ask on audiophile forums might actually clear this up a lot. I am an audiophile type and into expensive audio equipment. So I've done some reading on this stuff. However the exact functioning and signal dampening of the amplification stage to deal with the return signal is not something Ive delved into too much, as I'm sure you'd understand why. But the idea that the return signal has voltage and could technically be captured is one with merit.
Again, whether this is the case on all speakers, or on the pixels, I just wouldn't know and can't make any statement on. I would only say don't "assume" that it "can't" be done.