This is a seismic crisis -- a flurry of earthquakes overlapping in time -- that occurred in November, 2010 in the Gulf of Aden. The largest magnitude event was around magnitude 4.9. The earthquakes heralded a diking event on the mid-ocean ridge that goes through the Gulf of Aden and into the Red Sea. On account of the relatively short distance traveled by the seismic waves, they are still rich in high frequencies. Consequently, you hear sharp onsets for the P- and S-waves, and the occasional rising rumble of surface waves due to their dispersion. The longer the seismic waves travel, the more attenuated high frequencies become, so the sound shifts to lower frequencies.
The earthquakes were recorded by the seismic station near Addis Ababa, Ethiopia (FURI) at 8.9N 38.7E, about 670 km away. The seismogram is quite long: It lasts for nearly five days, and starts at 21:26 on Nov. 12. The first 45 seconds of the audio track are quiet and only feature background seismicity (audible events start around 22 and 33 seconds into the recording). At 03:03 on Nov. 14, the crisis begins (around 54 seconds into the recording). There are so many events that they pile up one on top of the other. A second burst begins at 10:26 on Nov. 15 (1:10 into the recording). Following this, the seismicity rate rapidly dies off, but maintains a high level of seismicity compared to the earlier level. As of 1 Dec. 2010, the seismicity rate is still above the level before the crisis.
Around the same time as the crisis, the lava lake at Erta 'Ale filled and overflowed. Whether the events are connected is unknown, but unlikely given their separation.