According to Appendix E of The Lord of the Rings, ⟨k⟩ is used with the
same value as ⟨c⟩ in names from non-Elvish languages (both representing
/k/). However, in the Silmarillion, ⟨k⟩ is also used in some Elvish
names, such as Tulkas and Kementári, as well as in some words in the
Appendix (Elements in Quenya and Sindarin Names), e.g. kir- as an
element or root in Calacirya, Cirth, and other words. And in earlier
versions of the language (when Quenya was called Qenya and Sindarin
Gnomish), ⟨k⟩ also often occurs. Therefore, let’s support it as an
alternative spelling of ⟨c⟩.

Currently, eSpeak NG doesn’t seem to do the two-step replacement of
⟨kh⟩→⟨ch⟩→⟨x⟩, which means that ⟨kh⟩ is ultimately pronounced as /kh/
(or /kʰ/?) rather than [χ]; according to Appendix E, this is correct in
Dwarvish, while in Orkish and Adûnaic ⟨kh⟩ should be equivalent to ⟨ch⟩.
Since we’re not really aiming for pronouncing any of these languages,
either way is fine.