Putting sand inside doors or walls made of wood tend to be bad after a few months or years because, as the sand settles down, they tend to sink and be concentrated in the lower part of the door leaving a gap on top. Furthermore, because of the weight of sand, as it settles on the lower portion of the door or wall, "nag-bubuntis" which will destroy your door or wall eventually. As the purpose of putting insulation material inside doors or walls is to prevent resonance and/or turn sound into heat energy, fibrous materials such as glass fiber, rock wall, etc. are better as they are lighter and stay in place through the years.
For doors, panel doors with insulation inside are mostly used. Gasketing like rubber seals and refrigerator magnetic seals are also used to great success. The problem with heavy doors is that they sag over time. Best solution? Construct a soundlock.
FWIW
