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 
);