Does anyone know if it is possible to implement playback of an audio file through the internal speaker even if the headphones are plugged in?
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
);