If it was new then you would definitely need it, unless it is vintage or handmade. Also, some products are made without a UPC.

A UPC is considered a GTIN but for the sake of making this post simple and to answer your question directly, I’ll just leave a link in case you want to read up on it.
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Yes, you need the UPC for your CDs even if the listing is used. If you do not have the UPC, Bonanza will automatically use the attribute identifier exists fale if you do not give the CD a UPC, but this does not mean that Google Shopping will like or even agree with this attribute for a particular listing. Thus, your listing may not appear in Google Shopping because of this. One main reason being is that if there are other ads in Google Shopping for the same product and they have a UPC, Google is going to know that product came with a UPC.

Based on your product, I would say that at the point of manufacturing the CDs, the used CDs had to of had a UPC at one point. So, yes, you need the UPC.

What you can do to find a UPC for your CD is go to www.google.com and type in the name of the CD along with UPC to see if any results come up for that CD. I bet you will be surprised.

For example, you’ve got a title to a CD in your store as Ted Nugent Catch Scratch. Go to www.google.com and type this in the search box and directly after the title type in UPC and there you go. You will see a UPC come up for this listing.

Of course this does not always work for every product but lots of times it does.

Here’s another up to date helpful link on GTIN.
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This is my take on a UPC when it comes to Google Shopping. If something I have said needs refining, then I am sure someone will post :)