http://www.blastam.com/blog/index.php/2013/03/how-to-track-downloads-in-google-analytics-v2/

track-outbound-download.js

if (typeof jQuery != 'undefined') {
    jQuery(document).ready(function ($) {
        var filetypes = /\.(zip|exe|dmg|pdf|doc.*|xls.*|ppt.*|mp3|txt|rar|wma|mov|avi|wmv|flv|wav)$/i;
        var baseHref = '';
        if (jQuery('base').attr('href') != undefined) baseHref = jQuery('base').attr('href');

        jQuery('a').on('click', function (event) {
            var el = jQuery(this);
            var track = true;
            var href = (typeof (el.attr('href')) != 'undefined') ? el.attr('href') : "";
            var isThisDomain = href.match(document.domain.split('.').reverse()[1] + '.' + document.domain.split('.').reverse()[0]);
            if (!href.match(/^javascript:/i)) {
                var elEv = [];
                elEv.value = 0, elEv.non_i = false;
                if (href.match(/^mailto\:/i)) {
                    elEv.category = "email";
                    elEv.action = "click";
                    elEv.label = href.replace(/^mailto\:/i, '');
                    elEv.loc = href;
                } else if (href.match(filetypes)) {
                    var extension = (/[.]/.exec(href)) ? /[^.]+$/.exec(href) : undefined;
                    elEv.category = "download";
                    elEv.action = "click-" + extension[0];
                    elEv.label = href.replace(/ /g, "-");
                    elEv.loc = baseHref + href;
                } else if (href.match(/^https?\:/i) && !isThisDomain) {
                    elEv.category = "external";
                    elEv.action = "click";
                    elEv.label = href.replace(/^https?\:\/\//i, '');
                    elEv.non_i = true;
                    elEv.loc = href;
                } else if (href.match(/^tel\:/i)) {
                    elEv.category = "telephone";
                    elEv.action = "click";
                    elEv.label = href.replace(/^tel\:/i, '');
                    elEv.loc = href;
                } else track = false;

                if (track) {
                    _gaq.push(['_trackEvent', elEv.category.toLowerCase(), elEv.action.toLowerCase(), elEv.label.toLowerCase(), elEv.value, elEv.non_i]);
                    if (el.attr('target') == undefined || el.attr('target').toLowerCase() != '_blank') {
                        setTimeout(function () {
                            location.href = elEv.loc;
                        }, 400);
                        return false;
                    }
                }
            }
        });
    });
}