iPhone audio playback: force through internal speaker?
I'm not sure how you are doing your audio playback, but try having a look at the "Redirecting Output Audio" section of the Audio Session Programming Guide
It looks something like this:
UInt32 audioRouteOverride = kAudioSessionOverrideAudioRoute_Speaker; // 1
AudioSessionSetProperty (
kAudioSessionProperty_OverrideAudioRoute, // 2
sizeof (audioRouteOverride), // 3
&audioRouteOverride // 4
);