The samples included with NSL Music Creator are a great way to experiment with the sequencer and put your first song together, but there are thousands of samples available online. Many are paid, but many are also free and have licenses which permit commercial use. You can also create your own samples for import into Unity. A couple of excellent free resources:
FreeSound.org is a massive repository of free audio samples. Be mindful of each sample’s licensing information. Many of these samples can be used in commercial products, but may require special attribution. Always review license information before incorporating samples into your project.
Audacity is an excellent and free sound editing application. It includes a large number of filters for manipulating audio, will allow you to record and blend multiple tracks, and is an all-around great tool to have on your workstation.