I actually got this working in my app the other day so I thought I'd share!
Using Unity 5 and Wwise 2014.1.4 - iOS integration package using a Mac for development
In the AkInitializer script
In the Awake function, find the line that initialises the platform settings;
AkPlatformInitSettings platformSettings = new AkPlatformInitSettings();
Then add this code somewhere after the above line......
#if UNITY_IOS && ! UNITY_EDITOR
platformSettings.eAudioSessionCategory = AudioSessionCategory.kAudioSessionCategory_AmbientSound;
platformSettings.bMuteOtherApps = false;
#endif