They're kinda "medium toned." Not "deep and gravelly" like dwarves. Not "high pitched melodic" like pixies. More a scratchy quality than "gravel."
Like a Tenor that's smoked too much. Oo! Or a late years Lucille Ball or Phyllis Diller. Kinda "deep for a human female"...but that's the males.
Edit: Found it! Derro Lore: The D&D Evil that Lives Beneath You
In a 1945 issue of Amazing Stories, Richard Sharpe Shaver claimed he had personal experience with a ancient and sinister civilization that exists in caverns beneath the earth. In 1932, Shaver noticed strange things were happening beneath the factory he worked in, things frightening enough that it forced him to quit his job.
The next several years of Shaver’s life is in dispute. Some claim that Shaver spent those years hospitalized for psychiatric problems.
Shaver himself swore he spent those years in the cavern world of the creatures he called “derro.”
As proof, Shaver produced a 10,000 wold document detailing an advanced alien race that he claimed built a vast subterranean civilization, only to abandon it for another planet...
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Richard Sharpe Shaver - Wikipedia
en.wikipedia.org