That is not exactly correct. Most panels absorb sound, converting it into tiny amounts of heat, so the energy in the sound wave is dissipated and removed from the room. this is opposed to a glass window, which reflects the sound wave and causes cancellations and reinforcements elsewhere in the room..
The very best sounding room I have ever experienced was a very small room in a high end dealership but the room was acoustically quite dead. The room sounded so much larger and the sound was clear and clean, without confusing reflections in the sound field. Whatever reverb and echo in the room was what was recorded, not what the room added. I also believe the dead room killed some distortions from the system, as they would not have as much energy in them as the music itself. I would treat my stereo room now other than the fact it would never pass the WAF.