We know that speed of sound is higher in a denser medium. So when a sound wave strikes a wall, why does it echo instead of passing through it with a speed faster than the speed of sound in air?
[Physics] Why is sound reflected and not transmitted through a wall
acousticsreflection
Related Solutions
The problem is acoustic impedance matching at the interface between the air and the wall. There an analogue of Fresnel's equations dictates how much power goes into the reflected and transmitted waves, and not surprisingly the more dissimilar the materials are the more power goes into the reflected wave. You can evade this by putting the speaker directly on the wall, effectively changing the problem so that the impedance matching is now between the source and the wall.
For a slightly surprising take on acoustic impedance, see this WP article.
Firstly, your point about total-internal-reflection (look into the tag wiki for further info about the phenomenon) resulting in 100% transmission of light energy and hence 100% intensity of incident light being preserved during reflection is conventionally considered true, but is not entirely correct.
Let me explain:
Reflection of any type, even if it is total internal reflection, cannot reflect 100% of the incident light energy. Total Internal Reflection nearly 100% (99.9999% maybe, but not 100%) efficiency compared to conventional reflection from a surface.
An interface between 2 optical media has the property of a "critical angle", the angle of incidence beyond which if light is incident on the interface from the optically denser medium into the optically rarer medium, it will be reflected back entirely into the denser medium, with no refraction into the rarer medium. Normally, at an interface between 2 transparent media of different optical densities, the light wave will be partially reflected into the medium from which it was incident and partially refracted into the medium into which it was incident, until the angle of incidence goes beyond this critical angle. This phenomenon of total internal reflection is usually observed for light, but is also applicable for other types of waves, like sound and waves on a string. This phenomenon is explained by Huygen's Wave Theory of Light in a classical sense. It occurs at the interface between 2 media of different densities in which the wave travels.
Hence, the intensity of incident light is preserved nearly entirely in total internal reflection, except for that minor fraction which may be absorbed by the denser medium itself, which is low due to the medium being transparent. There is also some energy loss due to photon tunneling across the interface, but again, it is not conventionally significant (total about $10^{-3} \%$). There are also evanescent-waves across the interface, but they do not result in net energy transfer across the interface. Refer Wikipedia here.
In conventional reflection, 2 surfaces are involved:
1) A transparent unsilvered surface
2) An opaque silvered surface
For light transmission through the unsilvered surface, similar energy losses are applicable as in case of total internal reflection.
However, for light reflection at the silvered surface, the surface being opaque, will absorb a significant portion (say about 1%-2%) of the incident light energy, while it will reflect most of the light energy due to it being silvered and reflective.
This 2nd significant energy loss of incident light at the silvered surface is not applicable for total internal reflection, hence we commonly say that total internal reflection forms images at 100% the brightness (intensity) of the incident light.
Best Answer
Whenever a wave reaches a boundary between two mediums some of the energy is reflected and some is transmitted.
The important parameter is not just the speed of the wave on either side of the boundary but what is called the acoustic impedance (= density $\times$ speed). If there is a large difference between the acoustic impedances then you will get most of the wave reflected.
So if you have an air (speed of sound 330 m/s, density 1.2 kg/m$^3$) brick wall (4200 m/s, 1850 kg/m$^3$) boundary a lot of the sound will be reflected.
So when you walk down a tunnel the sound that you emit gets reflected off the walls and even off the open end of the tunnel and comes back to you as an echo.