If you do this you will blow up your PI, as you apply both positive and negative voltages to the GPIO, and these voltages might be significantly greater than a GPIO input can withstand. Both negative voltages, and positive voltages greater than 4V, will damage a GPIO.
Question is, does the speaker have an adc circuit integrated into it?
NO! an ADC is an analog to digital converter, and no a speaker does not have an analog to digital converter built into it.....
even if you somehow would limit the speaker signal to a valid (safe) range. any audio you would be putting in would be unrecognisable distorted, as audio is analog, and all you do is clip it and make a digital signal by converting all positive voltages to ones, and all negative voltages to zero's that is NOT how audio sampling normally works.
P.S. split and renamed from unrelated hijacked ancient thread you "necromanced".