If your Sponsored Song campaign reaches its end date before the full budget has been spent, you can extend the campaign to complete the remaining funds.
To extend your campaign:
1) Click "Edit" on your campaign
2) Go to the third page of setup entitled "Budget"
3) Update your "End Date" to a future date, select "Save Campaign" on the top right, and your campaign will begin to spend again. If you reach your new end date and still have budget left over after this, you can complete the same process.