I've just been working on my own phone, have you set the volume on the SGT15000 object? Or if you have, set that to a higher value?
i.e.
AudioControlSGTL5000 sgt15000;
sgt15000_1.enable();
sgt15000_1.inputSelect(AUDIO_INPUT_MIC);
sgt15000_1.volume(0.5);
sgt15000_1.micGain(5);
On line 5...