The speed of sound is important because it tells us about the temperature of the Sun's plasma. The hotter the plasma, the faster the speed of sound.
The picture (right) shows that the predictions made before SOHO was launched were pretty good. Blue is slightly cooler and red is slightly hotter than we predicted but only by a very small amount of 0.1%.
That small difference in the centre of the Sun is very important - it suggests that the centre may be slightly cooler than we thought and so might not be providing quite so much nuclear fusion energy.