Modifying in-call voice playback in Android custom ROM

I would like to modify Android OS (official image from AOSP) to add preprocessing to a normal phone call playback sound.

I've already achieved this filtering for app audio playback (by modifying HAL and audioflinger).

I'm OK with targeting only a specific device (Nexus 5X). Also, I only need to filter playback - I don't care about recording (uplink).

UPDATE #1:

To make it clear - I'm OK with modifying Qualcomm-specific drivers, or whatever part that it is that runs on Nexus 5X and can help me modify in-call playback.

UPDATE #2:

I'm attempting to create a Java layer app that routes the phone playback to the music stream in real time.

I've already succeeded in installing it as a system app, getting permissions for initializing AudioRecord with AudioSource.VOICE_DOWNLINK. However, the recording gives blank samples; it doesn't record the voice call.

This is the code inside my worker thread:

// Start recording
int recBufferSize = AudioRecord.getMinBufferSize(44100, AudioFormat.CHANNEL_IN_STEREO, AudioFormat.ENCODING_PCM_16BIT);
mRecord = new AudioRecord(MediaRecorder.AudioSource.VOICE_DOWNLINK, 44100, AudioFormat.CHANNEL_IN_STEREO, AudioFormat.ENCODING_PCM_16BIT, recBufferSize);

// Start playback
int playBufferSize = AudioTrack.getMinBufferSize(44100, AudioFormat.CHANNEL_OUT_STEREO, AudioFormat.ENCODING_PCM_16BIT);
mTrack = new AudioTrack(AudioManager.STREAM_MUSIC, 44100, AudioFormat.CHANNEL_OUT_STEREO, AudioFormat.ENCODING_PCM_16BIT, playBufferSize, AudioTrack.MODE_STREAM);

mRecord.startRecording();;
mTrack.play();

int bufSize = 1024;
short[] buffer = new short[bufSize];
int res;
while (!interrupted())
{
    // Pull recording buffers and play back
    res = mRecord.read(buffer, 0, bufSize, AudioRecord.READ_NON_BLOCKING);
    mTrack.write(buffer, 0, res, AudioTrack.WRITE_BLOCKING);
}

// Stop recording
mRecord.stop();
mRecord.release();
mRecord = null;

// Stop playback
mTrack.stop();
mTrack.release();;
mTrack = null;

I'm running on a Nexus 5X, my own AOSP custom ROM, Android 7.1.1. I need to find the place which will allow call recording to work - probably somewhere in hardware/qcom/audio/hal in platform code.

Also I've been looking at the function voice_check_and_set_incall_rec_usecase at hardware/qcom/audio/hal/voice.c However, I wasn't able to make sense of it (how to make it work the way I want it to).

UPDATE #3:

I've opened a more-specific question about using AudioSource.VOICE_DOWNLINK, which might draw the right attention and will eventually help me solve this question's problem as well.

There are several possible issues that come to my mind. The blank buffer might indicate that you have the wrong source selected. Also since according to https://developer.android.com/reference/android/media/AudioRecord.html#AudioRecord(int,%20int,%20int,%20int,%20int) you might not always get an exception even if something's wrong with the configuration, you might want to confirm whether your object has been initialized properly. If all else fails, you could also do an "mRecord.setPreferredDevice(AudioDeviceInfo.TYPE_BUILTIN_EARPIECE);" to route the phone's built-in earpiece directly to the input of your recorder. Yeah, it's kinda dirty and hacky, but perhaps suits the purpose.

The other thing what was puzzling me that instead of using the builder class you've tried to configure the object directly via its constructor. Is there a specific reason why you don't want to use AudioRecord.Builder (there's even a nice example at https://developer.android.com/reference/android/media/AudioRecord.Builder.html ) instead?