Why does sound absorption in oceans depend on the pH?

Hard though it is to believe, pH does have an effect on sound absorption in water.

There are some reactions that are affected by pressure, that is pressure changes their equilibrium. One example is the equilibrium between boric acid and the borate ion:

$$ B(OH)_4\,^- + H^+ \rightarrow B(OH)_3 + H_2O $$

Increasing pressure pushes the reaction over to the right, and in doing so it absorbs energy. That energy comes from the energy in the sound wave, so unlikely though it may seem the chemical reaction absorbs sound. When the pressure is released the reaction moves back to the left, but the energy is released as heat not sound. The net result is sound energy gets converted to heat.

The problem with lowered pH is that as you lower the pH that moves the equilibrium to the right so there is less borate and more boric acid. That leaves less borate to absorb the sound. So lowered pH means less sound absorption.

If you're interested there is an article about this in Scientific American, and a more rigorous scientific publication here (sadly not available online). Lemon helpfully provided this link, Investigation of chemical sound absorption in sea water, and Googling should find you more relevant publications.

The effect is very small, not least because boric acid/borate concentrations in seawater are small. However sound propagates for huge distances in water - hundreds of miles. So even a small change in the water chemistry can have a measurable effect. The first of the papers I linked mentions that:

Sound attenuation in the low-frequency range is primarily due to boric acid relaxation and is a function of the seawater pH.

(my emphasis)

Whether that effect is actually big enough to deafen dolphins is a matter of some debate.