It is a known issue in Android OS itself. The delay depends on the Android version (newer versions tend to have smaller delay). In Unity we have done as much as we could and now all we can do is to wait for Google to make appropriate improvements in the Android OS. Check out the audio talk from Google IO 2013: https://developers.google.com/events/io/sessions/325993827
There Google's engineers explain the current situation of audio in Android, why it is like that and what is being done to improve it.