Combining mp4 and mp3 on Android with FFmpeg

I am developing a video capture application in Android. My goal is to merge the captured video file with the given mp3 file. I am using FFmpeg to merge files. The recording is done using the Android.Media platform. If now I try to combine the files as described here:

How to mux mp3 and mp4 files in Android

I get an error: avcodec_open2() error -1: Could not open video codec.

Is there a way to convert the captured video file to a readable version for FFmpeg? Or is there any other way to solve this problem?

i Don't want to shoot video with FFmpeg because it will be difficult and not clean (in my opinion)

Hope any body can help :) thanks in advance.

I Capture as shown in android DOC:

http://developer.android.com/guide/topics/media/camera.html

and this is how I am trying to combine

    FrameGrabber grabber1 = new FFmpegFrameGrabber(videoPath);
    FrameGrabber grabber2 = new FFmpegFrameGrabber(audioPath);
    grabber1.start();
    grabber2.start();
    FrameRecorder recorder = new FFmpegFrameRecorder(OutputPath,grabber1.getImageWidth(), grabber1.getImageHeight(), 2);
    recorder.setFormat("mp4");
    recorder.setVideoQuality(1);
    recorder.setFrameRate(grabber1.getFrameRate());
    recorder.setSampleRate(grabber2.getSampleRate());
    recorder.start();
    Frame frame1, frame2 = null;
    long timestamp = -2;
    int count = 0;
    boolean isFirstTime = false;
    boolean isFirstCheck = true;
    while ((frame1 = grabber1.grabFrame())!=null) {
        //frame1 = grabber1.grabFrame();
        frame2 = grabber2.grabFrame();
        recorder.record(frame1);
        recorder.record(frame2);
    }
    recorder.stop();
    grabber1.stop();
    grabber2.stop();
    }catch (org.bytedeco.javacv.FrameGrabber.Exception e) {
        e.printStackTrace();
    } catch (Exception e1) {
        e1.printStackTrace();
    }