When you reach a high score, a little ping sound is heard and the related high-score(s) briefly flashes.
If you would like, you can turn off this sound in your profile.
To do this, first click on My account.
Then, within your account you need to click on the Edit tab:
At the bottom there is a checkbox "Sound for a high score" where you can choose whether the sound is on or off:
Make sure you then click on the Save button: