The graphic above shows the trace of one hour's worth of data from Okmok's station OKCF.
The two bulbous features are the volcanic tremor, they sound vaguely like someone moving furniture.
Notice the The sharp line that occurs in the middle of the second tremor episode; this is a typical earthquake trace, and sounds like a crash or gunshot.

What you hear consists of about an hours worth of seismic data from Okmok's OKCF and OKWE stations, and has been sped up 100x.