Translation:I'm fed up; they talk only about politics.
Peter got a lot of up-votes because he's right. They sound exactly the same. This doesn't need a native. The phonetic transcriptions of il ne parle que and Ils ne parlent que are both [il.nə.paʁl.kə].
It is always the case that you can't distinguish singular and plural for regular 'er' verbs 3rd person present indicative (and some other tenses) unless there are vowels.
Elles jouent and elle joue sound exactly the same. Ils habitent is different from Il habite because of the liaison.
The French isn't negative. Ne .. que works grammatically somewhat like negatives (jamais rien personne plus aucun) but it's not negative.
Your translation means the same thing and is good English, but DL usually doesn't accept things that stray too far from a literal translation.
You're right about the liason and the c in Marc. As to et and ai, many speakers enunciate the difference between et (é) and ai (è) but in open syllables many don't and some regions say é for both. I asked my French teacher (from Normandy) about this and she confirmed that although she makes a distinction, many speakers don't. (Update: my current teacher, from the southwest of French, says he personally makes no distinction in these vowels in open syllables.)
That's a good question. Usually ne ... que doesn't follow the rule that de la becomes de when negative, since it's not really negative.
Google translate give Ils parlent de politique for "They are talking about politics." If that's correct, then perhaps this is one of those oddball cases where the French drop the article in specific idioms?
Maybe Jojo or one of the other French can clarify.