The audio files you found are for a function on

/static/src/js/im_chat.js

The part of the code you are looking for is starting from line 67:

 window_beep: function() {
            if (typeof(Audio) === "undefined") {
                return;
            }
            var audio = new Audio();
            var ext = audio.canPlayType("audio/ogg; codecs=vorbis") ? ".ogg" : ".mp3";
            var kitten = jQuery.deparam !== undefined && jQuery.deparam(jQuery.param.querystring()).kitten !== undefined;
            audio.src = openerp.session.url("/im_chat/static/src/audio/" + (kitten ? "purr" : "ting") + ext);
            audio.play();
        },

Good hunting.

SS*: all you have to do is just create a folder inside the im_chat module /im_chat/static/src/audio and add some audio files in mp3 or ogg format but remember to change audio file name to 'purr' or 'ting' or if you want to any other but some changes are required in js file /static/src/js/im_chat.js

So finally I have im_chat with audio notification which is very useful.

*SS - simple solution