Playing audio in iphone3 and iphone 4

Question

I did an iphone app that works on iPhone 3GS but not in iPhone 4. Here's the description of how it works:

1. First place a call to some one from iPhone
2. Put the call on speaker phone
3. Go to home screen of iphone
4. Launch my app that plays some audio
5. When the caller lifts the phone he will be able to hear the audio (if it goes to voice mail, it will record the audio)

I'm using AVAudioPlayer. I'm setting Audio session to playback mode. This is working on iPhone 3GS and the same functionality is not working on iPhone 4. What might be the problem.