This code should be useful for you:
#import <AudioToolbox/AudioServices.h>
UInt32 audioRouteOverride = kAudioSessionOverrideAudioRoute_Speaker;
AudioSessionSetProperty (kAudioSessionProperty_OverrideAudioRoute,
sizeof (audioRouteOverride),&audioRouteOverride);
It will increase volume. The functionality of the code is to convert the ordinary sound to speaker sound on ur iPhone. That's why kAudioSessionOverrideAudioRoute_Speaker is used.
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