Learn Dutch: Hun huis is sober, maar ze houden van eenvoud. - Their house is plain, but they love simplicity.

Not exactly! While it can mean 'not drunk' (though nuchter is much more common for that), in Dutch, sober usually means 'plain', 'austere', or 'modest' when describing things like a style, food, or a house.

This lesson is about noch ... noch (neither ... nor). How would I use that to say 'Their house is neither plain nor simple'?

You would say: Hun huis is noch sober noch eenvoudig. You couldn't use noch ... noch in the original sentence because it doesn't contain a negative contrast; instead, it uses maar (but) to connect two positive facts.

Why doesn't the verb houden go to the end of the sentence after maar?

Because maar (but) is a coordinating conjunction. Words like en (and), maar (but), and of (or) connect two main clauses without changing the word order. So, the second part of the sentence keeps the normal subject-verb order: ze houden.

I recognize een in eenvoud, but how does the word get its meaning?

Eenvoud means 'simplicity'. It literally translates to 'one-fold', similar to how 'twofold' works in English. You might already know its adjective form eenvoudig, which means 'simple' or 'easy'.