As a spelling convention, {wab Do} "speed of sound" is written as two
words. When used as a measurement term ("Mach"), it's written as one
word (wabDo). The pronunciation (and, for that matter, meaning) is the
same. (qepHom 2016)

The difference is whether a Klingon would perceive something as a single word or not.

In English we have the word desktop. As a noun, it refers to the surface of a desk. You can talk about a desk top as two separate words, describing the desk's top, but the actual name of the surface is desktop.

Now, English has rules about how to pronounce words that are lexicalized compounds versus productive phrases. When someone talks about the desk's top as desk top, the word top will be stressed. When talking about the name of the surface, desktop, the desk is stressed. This rule is regular in English: descriptive and ad-hoc phrases tend to be stressed at the end; fixed names tend to be stressed at the beginning. I once heard an explanation of this that used a clip from a Seinfeld episode: the characters were all talking about Chinese FOOD, but the expression has become fixed since then, and now we say CHINESE food. We say ICE cream, not ice CREAM. This feature of English lets us recognize lexicalized words just from hearing them.

The point is that there is a difference between lexicalized phrases and simply putting words together to describe a thing. In English we note it with stress. In Klingon we note it by spelling convention. Lexicalized phrases may be given to us as a single word, in which case we are justified in using it that way. But we cannot coin our own single words because that is equal to declaring the word as something that you will find in the dictionaries of Klingons. We don't have any of those to reference, so we cannot make any such assertion.

(I find it very amusing that my spell-checker doesn't have lexicalized in its database.)

-- 
SuStel
http://trimboli.name