The skeleton description says that while it cannot speak it, it can understand the languages it knew in life. Can it not speak because of its undeath or could it learn to speak a language?
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By RAW default, no.
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As explained by others it has no basic, default, means of expressing language. It's not about learning the language, it's the capability of the spell/energy powering the skeleton(s). Undeath is not in itself an impediment to speech (see: Liches).

Therefore you need to extrapolate a bit. The RAW description quoted above seems to imply the power source provides bare-minimum world interaction capabilities. And does not extend to advanced capabilities like mimicking a voice (disembodied or otherwise).

But, the "talking skull/skeleton" trope is incredibly common. So...

By customization, yes. (Possibly with the aid of a magic item.)
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It is your game, after all. And creativity is a core value of the gaming experience.

A skeleton is already magically animated, so if you wanted a **specific skeleton** to have the ability to speak, there are several creative ways you could explain the phenomenon.

* A magic item equipped by its creator, or even left over from its previous life. (An enchanted ring or amulet?)
* A slightly more powerful skeleton who gained the ability to speak over time via wild magic or the like.
* A skeleton created with extra riders on the creation spell by its creator. So maybe it was imbued with a secondary spell to allow a disembodied voice.

You could have all skeletons speak of that's how you want to run your world, but that'd be your own custom flavoring.

A speaking skeleton, otherwise, would be **special for some reason**.
<https://www.dandwiki.com/wiki/Skeleton_(5e_Race)> said

> 
> You've managed to control the energies that sustain you to allow you speak. You can speak, read and write Common, as well as the language of your creator. A Giant Skeleton may speak Giant, a God Touched may know Celestial, a Magic Fluke may randomly grant you a language and a Raised Minion may know their wizard master's draconic.
> 
> 
> 

so im guessing you can but i think it's for players thats race is skeleton