I understand how energy depends on amplitude as this source says but this source mentions that the energy of a sound wave depends (maybe "also on") on frequency of the sound wave. If energy of a wave depends on its frequency then won't that frequency reduce by a significant amount as the sound wave travels (the energy converts into other forms)?
The first link is nice, the second link points to a question which seems very confused to me. By absorption you just reduce the amplitude of the wave, not its frequency, which is conserved. It is however true that both these quantities determine the wave energy.
Another view of the answer can be in terms of sound particles, phonons. By absorption of sound, you reduce the number of phonons, not their individual frequency.
Note that conservation of frequency in linear time-invariant systems is a very fundamental rule, stipulated by the Noether's theorem.