DEVONthink doesn’t have any built-in support for text to speech. macOS does, so you may see it in some circumstances via the contextual menu > Speech or Edit > Speech. It does work on a limited basis but I don’t know that it would read entire documents except short ones. Apple’s site say you can “hear an entire document” but fails to define what a document is.
Also, a document may require a text selection to enable speech, e.g., PDF documents.