OK, so I beat the ODST campaign, and now I want to collect the audio logs. However, when I selected Mombasa Streets fromt he mission selects, most of the map was locked again. Is there a way to access my unlocked city, or do I need to play the game again to open it up again? Thanks.

Im just watching the credits now and am also wondering this (awesome game by the way)

Also, if I found one or two, are they going to reset when I start a new game?

I just tried this now and it would seem the answer is no, selecting Mombasa Streets presented me with a locked city as well, the only plus is that you keep all previous audio log progress with a new game so if you have 12/30 logs the next log you find in the next playthrough will be 13/30, but it seems starting over is the only way to continue the hunt.

after beating the game and returning to the menu i selected continue game and it placed me somewhere in the middle of the campaign. i don't know if it worked like this for you guys, but i imagine that i could run around and find them all at this point - if all 30 logs are available from the very start.

No you shouldn't have to start over. I was able to load right back into Mombasa Streets after beating the game and went cruising around on a Mongoose searching for logs.