public interface Service{
    @Streaming
    @Multipart
    @POST("/api/1.0/voice/audio")
    Call<ResponseBody> post(
            @Part("configuration") RequestBody configuration,
            @Part ("audio") RequestBody audio);
}

Content-Type = multipart/form-data;boundary=----------------------------41464684449247792368259
//HEADERS
----------------------------414646844492477923682591
Content-Type: application/json; charset=utf-8
Content-Disposition: form-data; name="configuration"
//JSON data structure with different audio parameters.
----------------------------414646844492477923682591
Content-Type: audio/wav; charset=utf-8
Content-Disposition: form-data; name="audio"
<audio_data>
----------------------------414646844492477923682591--

public RequestBody createPartForAudio(final byte[] samples){
        RequestBody requestBody = new RequestBody() {
            @Override
            public MediaType contentType() {
                return MediaType.parse("audio/wav; charset=utf-8");
            }

            @Override
            public void writeTo(BufferedSink sink) throws IOException {
                //Source source = null;
                sink.write(samples);

            }
        };

        return requestBody;
    }