What eventually becomes of the energy of sound waves?

Answer 1

Energy is neither created nor destroyed.

it simply become another form of energy.

While the air is necessary for the sound to travel, (any
medium that is made up of molecules will transmit sound, i.e. sound will
not travel in a vacuum), it also has inertia. Thus the sound wave must
"push" the air molecules into the next molecule and so on. The friction
between the molecules causes the sound wave to eventually lose its energy,
and thus its volume or intensity. All the while this is occuring, other
objects, animals and humans are absorbing the energy in the wave until the
original energy is absorbed into other systems in the form of mechanical
energy or chemical energy.

Answer 2

The energy is dissipated as heat, due to friction of the air molecules as they are disturbed by the wave. Same as water waves.

Answer 3

turns into heat.