Dull ache

Dull ache
 

Just to confirm

dull ache = dolor sordo ?

Yes. That is a direct translation.

Yes, as in "muffled" (obsolete now in English for deaf)

Both Oxford Superlex and Cambridge Klett bilingual use "sordo" as well, referring to an "ache".

Moliner gives,
D. sordo [o latente]. Dolor poco intenso, pero continuo y duradero.
And DRAE concurs with Moliner, (it also gives "dolor latente" = "dolor sordo")
~ sordo. 1. m. dolor que no es agudo, pero molesta sin interrupción.