Rockfalls and landslides are a common occurrence at many volcanoes around the world. The events may be small (single falling blocks), or very large avalanches. Triggers may be intense rainfall, rapid snowmelt, earthquake shaking, volcanic eruption, or water-level change.
This video is a composite of photos of a rockfall on Augustine volcano. The sound you are hearing is the seismic recording made audible. Notice how the sound is louder when there are surges in color at the volcano's summit.
Video courtesy of AVO.