OE
This is a complex sentence, in which the language, though, is not so much convoluted. The best strategy is always to cut the sentence you do have in front of you, especially if long like this one, into pieces.
The first sentence till the full stop is a full sentence in its meaning, standing on its own. It is a classic statement of fact: the room's design is strictly related to the purpose it has, following certain standards; in this case, the music or else.
The second one sentence is an example to show up what is the real concept behind the first sentence. Music, for instance, is enjoyed or appreciated in certain space but this same space is not suitable for speech. Notice here the shift in the argument. This is typical of the test makers to do so. Rarely a sentence with one blank up to 3 goes in one direction. There is always a sort of twist.
Now, we need a word in somehow in contrast with the first word: in this case, the meaning is that speech in a room built for music is LESS understandable. The only choice that fits the scenario is intelligible, which means less understood.
The last sentence, instead, starts telling us that a third way is possible: in other words a room built in such way to serve both of the purposes: music and speech. As such, the manner via which the music and speech are expressed in the same room must be compromised. We use the standards aforementioned at the beginning of the sentence to serve the scope, in a double way.