They cannot propogate space you are right, but these sound waves are those that travel through the Suns interior. They travel down into the inside of the sun, and are caused to refract by the changes in the speed of sound the deeper they go. Eventually they end up heading back towards the surface where they will rebound back into the sun if below a critical frequency. This frequencies become ‘quantised’, a posh physics word that essentially means only set values of frequency waves can exist, all others decay.
Now, they allow us to draw conclusions on the sun due to their dependence on the suns composition and behaviour. So, as sound speed depends on density, we can determine the density structure of the sun. Also, due to the ‘quantised’ structure of the waves, special ‘multiplet’ waves are produced at a small frequency separation from the main waves which are dependent on the strange rotation of the sun. This has allowed us to map the full rotation pattern of the suns convective zone and understand the suns dynamo.
We can obtain all this information simply through observing the suns surface. We take the wave pattern we observe and the distribution of frequencies and compare them to those that a theory predict. Tweaking of the theory then leads us to finding the model that best describes the sun. No other method allows us to understand the suns interior so well!