I realize this question is fairly old but when I was struggling with the same problem I found a simple solution that hopefully will help anyone else looking to use the louder media speakers as opposed to the receiver speakers. The method I used was setting up the audio session with the DefaultToSpeaker option in AVAudioSessionCategoryOptions:
In Swift (assuming your audio session is named session) –
session.setCategory(AVAudioSessionCategoryPlayAndRecord, withOptions:AVAudioSessionCategoryOptions.DefaultToSpeaker, error: nil)
In Obj-C –
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord withOptions:AVAudioSessionCategoryOptionDefaultToSpeaker error: nil];