Translation:Your neighbor Omar is a translator, Rawad.
In Standard Grammar (Arabic fuSa) for the sentence above, the complete ending sounds will be:
جارُكَ عمرُ مرتجمٌ، يا روادُ.
"jaaruka 3umaru murtajimun yaa rawaadu."
So, actually all words have ending sounds, which you called it as "extra sounds" (-un, -an, -in, etc.), which are the main subject of "النحو" -- ie. a part of Arabic that studies the ending sounds (or we can say "Arabic Grammar").
However, in the daily conversation, those ending sounds are oftentimes omitted by native speakers. And, ... as Duolingo also tries to omit all endings (similar to Arabic that is practiced on daily basis), the "extra sounds" that you have heard are probably an audio glitch.
Note: "جارَك" jaarAk that Duolingo has tried to apply is a Slang/Dialect -- and not Standard Arabic. In the Slang, people never spell the ending sounds.