How do I overlap sound files and save it

Question

I'm trying to create a karaoke system in Java, I have a music MP3 file and a file which I recorded by singing, I want them to overlap and save it as another file., So how do I do it?

I tried writing a buffer from one file and the second from the other file and kept it on repeat till the end of the file but It didn't work out I also tried concatenating them but the program just added the second file at the end of the first file and saved it as a third file, I didn't work either.

        File f1 = new File("D:/Background OST 2.MP3");

        InputStream is = new FileInputStream(f);
        InputStream is1 = new FileInputStream(f);

        OutputStream outstream = new FileOutputStream(new File("D:/blabla.MP3"));


        byte[] buffer = new byte[4096];
        byte[] buffer1 = new byte[4096];
        int len = 0;
        int len1 = 0;
        int index = 0;
        boolean complete = false , complete1 = false;

        while(true){
            if(!complete){
                if((len = is.read((buffer))) > 0){
                    outstream.write(buffer, 0, len);
                }else{
                    complete = true;
                }
            }

            if(!complete1){

                if((len1 = is1.read((buffer1))) > 0){
                    outstream.write(buffer1, 0, len1);
                }else{
                    complete1 = true;
                }
            }
            byte b = -1;
            if(buffer.length > index){
                if(buffer[index] > 0){
                    b = buffer[index];
                }
            }

            if(buffer1.length > index){
                if(buffer1[index] > 0){
                    b += buffer1[index];
                }
            }

            //outstream.write(b, 0, len1);
            outstream.write(b);

            if(complete == true && complete1 == true){
                break;
            }

            index++;
        }

        outstream.flush();

        // closing resources
        is.close();
        is1.close();

        outstream.close();

I expected the code to overlap a music file and another file that I recorded via mic but Instead of that the new file concatenate the sounds and there were too many weird noises in it.