Well, I don't mean to be mean.. But downloading songs for free is piracy isn't it? BlackBerry isn't exactly the best platform for non-secure practices.
If you want to buy songs, there is BlackBerry World and if you're on the latest leaked OS, you can install Amazon and buy some from there too, but getting songs for free, you'd end up having to resort to whatever method you would on a PC (and use the browser).
That all said, some bands fo release albuns for free every now and then! I'd keep an eye out for those too!