Speed of sound in thin long solid bars is given by
modulus for rubber Y = 4 * 10⁶ N/m² and density ρ = 1.2 or so.
sound in rubber using the above formula = 57.7 m/sec. This is much smaller than the
speed of sound in air, which is 340 m/sec at atmospheric pressure/Temperature.
For vulcanized rubber the Young's modulus Y = 2.8 * 10⁶ N/m² and the density = around 920 kg/m². So speed of
sound = 55.1 m/sec.
Vulcanized rubber is even softer than rubber. So
the pressure changes in it, due to sound waves are not much. The particles in
vulcanized rubber do not vibrate or oscillate as fast as in other solids or in
propagates only if the particles of the media oscillate. Then there are pressure differences that propagate. Vulcanized Rubber
particles are very soft and they compress and expand side ways. They absorb
some pressure from the sound waves. The vibrations of the particles is not
perfect and elastic. It is reduced as compared to hard solids
Hard metallic objects, if you hit it, it vibrates for a long time. Soft objects like rubber regain their shape back quickly and stop vibrating quickly. That property is the one that makes velocity of sound more in hard metals and less in rubber.