Socket.io doesn't correctly attach to http server in express application

I'm pretty new to NodeJS and trying to get Socket.IO running with an Express application. I've tried to stick to the docs and tutorials as close as possible, but in the end, socket.io.js cannot be found on client side.

The server is being started as follows:

var app = require('../app');
app.set('port', process.env.PORT || 3000);

var server = app.listen(app.get('port'), function() { debug('...') } );

Routing/serving pages:

var express = require('express');
var router = express.Router();

router.get('/', function (req, res) {
    res.render('index', { title: 'Express' });
});

Including socket.io:

var express = require('express');
var http = require('http').Server(express);
var io = require('socket.io')(http);

And in the HTML file, for clients:

<script src="/socket.io/socket.io.js" type="text/javascript">

But when loading the page, this JS-file is never found. I suspect that including socket.io via that previously created http server is somehow wrong... Or am I missing something else?

Update

I just found out that the snipped that includes socket.io is executed before the server is started (first snippet). They're in different files and the server-start one (named www.js) is configured as "start" script in package.json, the other one (websockets.js) required indirectly by that.

If I put

var io = require('socket.io')(server);

at the end of www.js, everything works just fine. But is there a way to do it from websockets.js, which is loaded before, e.g. by a server start callback function?