I want to download music software but it wont let

What you need to do is set your software repositories. Go into Synaptic Package Manager and select Repositories from the Settings Menu. In the Ubuntu Software tab, select all of the checkboxes under the "Downloadable from the Internet" heading. For good measure, you might also want to select all the checkboxes under the Third-Party Software tab as well. This should allow you to download more than just the purely open-source/GNU software that is usually enabled by default.