For a person's voice:
* Whimpering: This suggests a low, soft, and often unsteady sound made by someone who is sad or afraid.
* Sobbing: A loud, uncontrolled crying sound, often accompanied by gasping for breath.
* Moaning: A low, mournful sound expressing pain or sadness.
* Groaning: A low, deep sound expressing pain, discomfort, or sadness.
* Weeping: This suggests a quieter, more tearful form of crying.
For sounds in general:
* Mournful: This describes a sound that evokes sadness or grief.
* Melancholy: This suggests a wistful, sad sound.
* Despondent: This describes a sound that expresses hopelessness or despair.
* Hollow: This suggests a sound that is empty and devoid of life, which can be associated with sadness.
* Dull: This describes a sound that is lacking in brightness or energy, and can be associated with depression.
The best word to use will depend on the specific context and the type of unhappiness you want to convey.