"We talked about his address."
Translation:Мы говорили о его адресе.
Why do we say "о его" and not "об его"? I thought you say об before a word that begins with a vowel.
The first sound in the word "eго" is actually a consonant: "й". Remember, Russian vowels е, ё, ю, я are actually pronounced with a leading "й" whenever they appear at the beginning of a word (or after another vowel, ь ор ъ).
So "говорить о..." demands prepositonal case? I wanted to use Genitive "адреса".
I was also wondering about this. Also should we say него instead of его after о?
Not only. "Счёт" literally means "account". E.g., "банковский счёт"="bank account". And just like English "account" ("on account of something", by your own account etc.), Russian "счёт" has less literal uses. Specifically, "на счёт чего-то [genitive]" means "on the subject of something". Hence in this particular case, you could actually use this expression, but Max84370 used the wrong case for "адрес" (and also missed "его"="his").
"Мы говорили на счёт его адреса" could be an acceptable answer.