You'd have to use regex to match the URLs within the string, and then wrap them in an anchor:
$('p:contains("http://")').html(function(index, html) {
var url = html.match(/(((ftp|https?):\/\/)[\-\w@:%_\+.~#?,&\/\/=]+)|((mailto:)?[_.\w-]+@([\w][\w\-]+\.)+[a-zA-Z]{2,3})/g);
$.each(url, function(i, v) {
html = html.replace(v, '<a href="' + v + '">' + v + '</a>');
});
return html;
});
The example above uses the regex pattern found in this answer