Researchers use careful measurements of how the Sun's plasma rises and falls to detect up to a million different sound waves echoing through its gases. Sunspots rise and fall in frequency over an 11-year cycle, signalling peaks and lows in solar activity.
The original activity developed by Frank Hill and colleagues at Kitt Peak: written as a script for astronomers visiting classrooms to introduce "helioseismology" to grades K-3 (!) And with its engaging musical approach, it works!
Stanford Solar Center: Michelson Doppler Imager, MDI, one of the instruments on SOHO. Site provides images and results obtained from MDI.
How does the Sun sound?: Visit this site to find the answer.
Global Oscillation Network Group (GONG): contains the latest information the international GONG group has gathered through solar helioseismology. Follow the helioseismology link for additional background.