You'd use Novocaine to grab the audio, and then you'd do the audio format conversion yourself. e.g.:

[[Novocaine audioManager] setInputBlock:^(float * inData, UInt32 numSamples, UInt32 numChannels) { for (int i=0; i < numSamples * numChannels; ++i) { dataForVoip[i] = YourType(inData[i]) * someScaling; } }];