Most PA systems drive their speakers using a "100V Line" system with transformers. Either way, the characteristic impedance of the speaker system will be sufficiently low enough for it not to be affected by mains "noise". Also there will be no "grounding" of either speaker cable core, making it a "balanced" signal, so any induced current from the ac wiring will be cancelled out at the receiving end.
Does that change people's views on the subject.
I have never heard of any issues regarding crosstalk between mains cables and speaker cables, and very few installations even attempt to keep the two sets of wiring apart.
You could, of course, elect to use shielded cable for your PA speakers, making the job more expensive.
And if you are "direct burying" 110V ac cables, shouldn't those be armoured, providing another way of shielding.
EDIT : In any case, if the speaker cables run anywhere near the 110V ac cables, they must have a voltage rating equivalent to, or greater than, the rating of the 110V circuit.
2nd EDIT : I believe that if you separated the buried cables by 100mm, which can be done either side-by-side, or "over-under", depending on the width of your trench, you will eliminate the possibility of any "substantial" pickup. The 12ft runs inside the poles would be less significant, and probably negligible. And if you can maintain some degree of separation, even better, depending on the pole diameter. Even if you can only get 10 mm, that would be better than none. Don't forget that you will only need 1 cable to each PA speaker, use insulated junction boxes below ground level. Otherwise you'd have to run two, loop-in, loop out.