The sounds from the other room could mostly be heard because of:
The vibration of molecules in the substance of the wall
CONCEPT: Transmission through a wall?
The scientifically accepted model of sound based on a wave model is that the transmission of sound through a wall is due to the vibration of molecules - in this case, the molecules that constitute the solid wall.
Although the propagation of sound in air is generally understood by students, they have difficulties in explaining the transmission of sound through a solid wall.
Previous studies have found that students do not seem to use the same reasoning to explain the same phenomenon: the transmission of sound in air or through a solid wall.
Students seem to perceive transmission through air versus a wall as being two entirely different phenomena.
Source: Q6,7 & 8: http://baldufa.upc.edu/recerca/articles/a42/eaeeie_2009_misconceptions_sound_waves.pdf
As a sound wave travels through a medium, it will often reach the end of the medium and encounter an obstacle or perhaps another medium through which it could travel. When one medium ends, another medium begins; the interface of the two media is referred to as the boundary and the behaviour of a wave at that boundary is described as its boundary behaviour.
The amount of energy that becomes reflected is dependent upon the dissimilarity of the two media. The more similar that the two media on each side of the boundary are, the less reflection that occurs and the more transmission that occurs. Conversely, the less similar that the two media on each side of the boundary are, the more reflection that occurs and the less transmission that occurs. So if a heavy rope is attached to a light rope (two very dissimilar media), little transmission and mostly reflection occurs. And if a heavy rope is attached to another heavy rope (two very similar media), little reflection and mostly transmission occurs.
http://www.physicsclassroom.com/class/sound/Lesson-3/Boundary-Behavior
