Speakers pass 80v on the 3.5mm plug, is it safe to use it on motherboard/display 3.5mm jack?

Depends on where that "80V" is actually coming from and why. If the speakers use a switching power supply and a power cord without ground pin, the 80V could simply be leakage current from the Y-class capacitors and would be normal. If the speakers have a ground pin on the power cord, having a significant AC voltage would seem to indicate a grounding fault either within the speaker amplifier, power cord or building wiring. If the speakers use a traditional AC transformer, the leakage should be negligible and not register anywhere near 80V, which may imply an insulation failure of some sort somewhere.

Having no external brick doesn't prove that it isn't using a switching supply, it can be internal just like it is on most TVs, monitors, PCs, etc. External power bricks aren't necessarily switching supplies either, though plain external AC transformers have become far less common over the past 10 years in large part due to the rising cost of copper and increasing availability of inexpensive monolithic switching regulators. Based on how the sub weighs only four times as much as the speakers despite being about four times as large itself, that doesn't leave much weight headroom to fit an AC transformer, so I'm going to bet on an internal switching power supply.

I doubt a HV power line would have enough of an electric or magnetic field to couple 80V into a 5' wire at 30+'. Maybe into a 100' wire roll. Even if it did, that 80V would have practically no current behind it and collapse to ~0V the moment you connect it to anything grounded: if the 80V you are reading does indeed come from magnetic induction from power lines, then only ~8uA are passing through the ground loop created by your meter and once you hook it up to something with much lower impedance than your meter, the voltage will drop well under 1mV.

There is a simple way to test this: unplug your speakers' power cable and see if the 80V is still there. If it is gone, then you know that the voltage comes from the speakers' power supply. If it is still there, then you know it comes from the environment.