AEI laymans summary of the following paper:
Hester, Peltzer, Kirkwood, Brewer. Unanticipated consequences of ocean acidification: A noisier ocean at lower pH. Geophysical Research Letters, Vol. 35, L19601.

This is a theoretical, rather than field research, study that calculates the likely current and future decreases in sound absorption caused by increasing ocean acidity (lower pH). The bottom line result is sobering for anyone who is already concerned about the rising tide of ambient noise in the world’s oceans. Increasing shipping noise, in particular, is reducing the effective communication ranges of great whales and creating an urbanized environment in many coastal areas. This research suggests that the well-documented increases in ocean acidification are already helping sound to travel further, with dramatic increases likely in coming decades.

The paper considers four causes of increasing ocean acidity, including deposition of CO2, Nitrogen, and Sulphur, and some chemical effects of warming (which itself contributes in a much smaller way to decreased sound absorption). The net result is that it appears likely that low- and mid-frequency sound absorption has already decreased by 10-15% as ocean pH has gone down by .12; this means that sound travels 10-15% further, with the effect that background ambient noise levels (predominantly wave noise and shipping noise at the frequencies being impacted) rise as the cumulative noise in any one place contains source noise from a larger area. The most dramatic impact is in sound below 1kHz, with significant effects up to 10kHz. Projections for the continued acidification and therefore furher increases in sound propagation suggest that by 2050, we will easily hear a 30% increase in sound propagation distances (based on a low-end prediction of a decrease in pH of .15); more likely is a 40-60% increase in sound propagation distances at 1kHz with pH decreases of .30 to .45. Meanwhile, the models suggest a corresponding 20% increase in propagation of 10kHz sounds in all pH scenarios.