Yeah don’t worry at all it is probably a ring tone. Let me keep this simple, your computer has a sound built into it called “cha Ching” You have nothing to worry about it was probably an incoming email with the default sound. To change the sound go to your sound settings of system preferences and let apple’s amazing user interfaces take you to your worrying sound.
Overall have no worry’s. You pr computer is a state-of-the-art machine and no purchases were made. PS by analysing your report it sounds like when you heard the sound your computer screen was tilted upward, so don’t be surprised when you hear and sound and not see the screen light up. It is just to save power and be easier on your 13 inch screen of yours.
Have a great day and enjoy your morning papers.