In my linguistics courses (at a university in the United States), professors pronounced "marked" as a single syllable.

Generally, when indicating we want to give an E that is normally silent its own syllable, we use the grave accent. E.g., for "marked" we would write "markèd".

At least in General American dialects, "marked" is rendered as /markt/ because of rules about vowel epenthesis and consonant voicing assimilation. /markd/ would indeed sound odd to the American ear, I think. But when you explicitly add the vowel back in, the voiceless /k/ no longer forces the /d/ to assimilate its voicing characteristic, so you get /'mar.ked/. (I am using informal IPA because I'm too lazy to go copy/paste the specific symbols, but in this context they would add little anyway.)