https://magpcss.org/ceforum/viewtopic.php?f=17&t=17715&p=46166#p46166

ClientSchemeHandler.java

// Copyright (c) 2014 The Chromium Embedded Framework Authors. All rights
// reserved. Use of this source code is governed by a BSD-style license that
// can be found in the LICENSE file.

package com.codename1.impl.javase.cef;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

import org.cef.callback.CefCallback;
import org.cef.handler.CefResourceHandlerAdapter;
import org.cef.misc.IntRef;
import org.cef.misc.StringRef;
import org.cef.network.CefRequest;
import org.cef.network.CefResponse;

/**
 * The example for the second scheme with domain handling is a more
 * complex example and is taken from the parent project CEF. Please
 * see CEF: "cefclient/scheme_test.cpp" for futher details
 */
public class ClientSchemeHandler extends CefResourceHandlerAdapter {
    public static final String scheme = "client";
    public static final String domain = "tests";

    private byte[] data_;
    private String mime_type_;
    private int offset_ = 0;

    public ClientSchemeHandler() {
        super();
    }

    @Override
    public synchronized boolean processRequest(CefRequest request, CefCallback callback) {
        boolean handled = false;
        String url = request.getURL();
        
        // Using https://file-examples.com/wp-content/uploads/2017/11/file_example_MP3_700KB.mp3 downloaded locally
        // as a test file
        File f = new File("/Users/shannah/tmp/test.mp3");
        try (FileInputStream fis = new FileInputStream(f)){
            data_ = new byte[(int)f.length()];
            fis.read(data_);
            mime_type_ = "audio/mp3";
            handled = true;
        } catch (IOException ex) {
            ex.printStackTrace();
        }
        


        if (handled) {
            // Indicate the headers are available.
            callback.Continue();
            return true;
        }

        return false;
    }

    @Override
    public void getResponseHeaders(
            CefResponse response, IntRef response_length, StringRef redirectUrl) {
        response.setMimeType(mime_type_);
        response.setStatus(200);

        // Set the resulting response length
        response_length.set(data_.length);
        System.out.println("Response length: "+data_.length);
    }

    @Override
    public synchronized boolean readResponse(
            byte[] data_out, int bytes_to_read, IntRef bytes_read, CefCallback callback) {
        boolean has_data = false;

        if (offset_ < data_.length) {
            // Copy the next block of data into the buffer.
            int transfer_size = Math.min(bytes_to_read, (data_.length - offset_));
            System.arraycopy(data_, offset_, data_out, 0, transfer_size);
            offset_ += transfer_size;
            System.out.println("Read "+transfer_size+".  Offset now "+offset_);
            bytes_read.set(transfer_size);
            has_data = true;
        } else {
            offset_ = 0;
            bytes_read.set(0);
        }

        return has_data;
    }

    
}

log.txt

 [java]      [java] Response length: 764176
     [java]      [java] Read 65536.  Offset now 65536
     [java]      [java] Read 65536.  Offset now 131072
     [java]      [java] In shouldNavigate callback with url: {"duration":27252}
     [java]      [java] In shouldNavigate callback with url: {"state":"playing"}
     [java]      [java] In shouldNavigate callback with url: {"time":32}
     [java]      [java] In shouldNavigate callback with url: {"time":189}
     [java]      [java] In shouldNavigate callback with url: {"time":441}
     [java]      [java] In shouldNavigate callback with url: {"time":701}
     [java]      [java] In shouldNavigate callback with url: {"time":933}
     [java]      [java] In shouldNavigate callback with url: {"time":1183}
     [java]      [java] In shouldNavigate callback with url: {"time":1434}
     [java]      [java] In shouldNavigate callback with url: {"time":1686}
     [java]      [java] In shouldNavigate callback with url: {"time":1943}
     [java]      [java] In shouldNavigate callback with url: {"time":2134}
     [java]      [java] In shouldNavigate callback with url: {"time":2134}
     ...
     [java]      [java] Exception in thread "AWT-AppKit" [0709/085805.325138:ERROR:batching_media_log.cc(38)] MediaEvent: {"error":"FFmpegDemuxer: data source error"}
     [java]      [java] [0709/085805.327419:ERROR:batching_media_log.cc(35)] MediaEvent: {"pipeline_error":9}
     [java]      [java] In shouldNavigate callback with url: {"error":"PIPELINE_ERROR_READ: FFmpegDemuxer: data source error. Code=2"}
     [java]      [java] In shouldNavigate callback with url: {"time":2197}
     [java]      [java] In shouldNavigate callback with url: {"state":"paused"}
     [java]      [java] [EDT] 0:0:44,524 - PIPELINE_ERROR_READ: FFmpegDemuxer: data source error. Code=2

test.html

<!doctype html>
<html>
  <body>
    <audio src="client://tests/streams/1" controls autoplay/>
  </body>
</html>