How do I add a "Buy" button to my sounds?

You can add a Buy button to your tracks and playlists by going to their edit page through the pencil icon below their waveform. In the edit page, go to the 'Metadata' tab and add the external link to where your track can be purchased:

Once you click 'Save', a "Buy" button will be rendered on the player like this:

If you have a Pro or Pro Unlimited subscription, you can also customize the text used for the Buy button on the edit page in the 'Buy link title' section.