Is it possible to send a byte array in a json over socket.io java client

this is actually two questions in one but they are closely related

I have read plenty of times that you have to send either json/string or binary data over a websocket like socket.io but that you cannot mix these types. But then I was puzzled by finding the following example in the official documentation of the socket.io client java implementation

// Sending an object
JSONObject obj = new JSONObject();
obj.put("hello", "server");
obj.put("binary", new byte[42]);
socket.emit("foo", obj);

// Receiving an object
socket.on("foo", new Emitter.Listener() {
  @Override
  public void call(Object... args) {
    JSONObject obj = (JSONObject)args[0];
  }
});

where the "binary" element of that json is clearly binary as the name suggests. The documentation talks about socket.io using org.json, but i couldnt find that this library supports adding binary data to json files anywhere.

is this functionality now supported? if so, what is socket.io doing in the background? is it splitting the emit in two separate messages and then remerging it? or is it simply saving the binary data in base64?

A bit of background.

I am trying to add a private chat functionality to my app so that a user can have multiple private two-party audio-message based chat conversations with several other users. i am having issues finding out how to tell my server how to forward all the messages. if i use a json i can simply add a sender and a receiver to the json and have the server read the id of the receiver and forward the message accordingly. but i am not sure how to handle messages containing only binary data. i have no idea how to add metadata (such a sender and a receiver id) to them so that the server knows to whom they are addressed. i have heard the suggestion of sending a json with a sender id, a receiver id and an MD5 hash of the file i am trying to send, and then send the binary data alone separately and having the server match the two messages over the md5 signature, but that seems to come with problems of its own. like i dont know how having to calculate the MD5 of a ton of audio files on the server is going to affect server performance and there is also the issue of potentially receiving the audio byte array before the json specifying its destination has arrived.

there is always the alternative of encoding my audio files in base64 and sending them as json, as i have been doing so far, but i have been told this is a bad practice and should be avoided as if inflates package sizes.

i feel like there are a bunch of messaging apps already out there, and i bet at least some are based on websockets. I would like to know if there are any best practices on how to route binary data over a websocket to a specific receiving connection.

Ill upvote any answer to the questions above as well as any hint on how to tackle the problem mentioned in the background part.