Learn Farsi: هر استاد در خصوص این میزگرد صحبت می‌کند. - Every professor speaks regarding this roundtable.

dar khosus-e is a bit more formal and translates well to 'concerning' or 'in regard to'. While you could replace it with dar moured-e or râje' be (meaning 'about'), dar khosus-e perfectly matches the academic tone of a professor and a roundtable. Notice that like dar moured-e, it requires the spoken ezâfe (the 'e' sound) at the end to connect to the following noun.

Why is ostâd singular here? Shouldn't it be plural if it implies multiple people?

Just like in English, the Farsi word har (every/each) must always be followed by a singular noun. You will say har ostâd (every professor), never the plural har ostâdhâ. Following this rule, the verb is also conjugated in the singular third-person form (mikonad).

The word for roundtable, mizgerd, looks like a combination of two words. Is it?

Yes, it is a compound noun made of miz (table) and gerd (round). When Farsi forms compound nouns like this, it often fuses a noun and an adjective into a single word without using the ezâfe connector. It is used exactly like the English word 'roundtable' to mean a panel discussion.

Does sohbat mikonad mean 'speaks' or 'is speaking'?

It can mean either! The Farsi present tense with the mi- prefix covers both the simple present (habitually speaking) and the present continuous (currently speaking). You would rely on the wider context of the conversation to know whether this roundtable discussion is a general fact or happening right now.