If you want to do another recording before you quit the program, you need to discard the tracks from the recording you just did.  Click the “X” in the box at the upper left of the tracks window.  This discards the track and leaves Audacity ready to make another recording.

 

Since you've made MP3 versions of the recordings, there's no need to save the document.  When you quit, you may safely answer "No" when asked if you want to save.  However, there's no reason you shouldn't save the project locally on your hard disk if you prefer.