When in an apartment or other places with walls, radegast users can hear through them. I am told that by people that use Firestorm or the regular viewer that this is not the case for them. This isnt anything major but just thought I'd mention.
Go to a place, such as an apartment that has walls. If their is something else next to it, such as a fishtank or something else that makes noise in an adjacent apartment you will be able to hear it. I am told by other people that do not use this client that they can't hear that, only what is in their own apartment. We can hear any sounds, from any items to chat that is happening.
What you've been told is incorrect. The Second Life server platform does not make any distinction between types of objects for the purposes of sound or chat. While it may look like a wall, there is no such thing as a "wall" as far as the engine is concerned.
Yes, this is how the SL platform works, not something clients like Radegast can change.