We show that ocean acidification from fossil fuel CO2 invasion and reduced ventilation will result in significant decreases in ocean sound absorption for frequencies lower than about 10 kHz. This effect is due to known pH-dependent chemical relaxations in the B(OH)3/B(OH)4− and HCO3−/CO32− systems. The scale of surface ocean pH change today from the +105 ppmv change in atmospheric CO2 is about −0.12 pH units, resulting in frequency dependant decreases in sound absorption (α = dB/km) exceeding 12%. Under reasonable projections of future fossil fuel CO2 emissions and other sources a pH change of 0.3 units or more can be anticipated by mid-century, resulting in a decrease in α by almost 40%. Ambient noise levels in the ocean within the auditory range critical for environmental, military, and economic interests are set to increase significantly due to the combined effects of decreased absorption and increasing sources from mankind's activities.