osCommerce does have "digital download" capability that works as you describe. As for the previews/samples, in the product description just include a link to a suitable format sample file (.mp3, etc.).

If you're on osC version 2.2, it's something like Admin > Configuration > downloads set to true, and I think you have to mark each downloadable product as such. If you're on version 3 (beta), it's probably similar. Go to forums.oscommerce.com, Community > osCommerce Online Merchant v2.x > General Support and search for "digital". Apparently there are some complications with certain payment systems, such as PayPal IPN, so read up on it carefully. There's also a "Super Download" mod (contribution) that may be of interest.

I haven't played with digital downloads myself on osCommerce, so I would strongly suggest that you go over to their site and read the documentation and search the forum for other questions and answers on this subject.

For a "preview" (sample?), it's possible that the "Super Download" contribution may help you with that. I'm not familiar with it, so if not, you'll have to create a directory somewhere with the uploaded samples, and in the product description you just put a link (<a href=...) to that directory and file.