I use IP Cam Viewer on both my iPhone and iPad and it has 2 way audio to my 8918 cameras. There is also an Android version which also claims to have 2 way audio, but I obviosly cannot confirm that.
What I like is the fact that it is a multi-camera app, but you can choose how many cameras to display in the front page. A click on any camera display will then display that camera full screen. Its the best app for me out of about 10 Ive tried.