Can't Pronounce S with Invisalign?

First, it is not uncommon for your speech to be affected during active Invisalign treatment, especially with the aligner trays in your mouth.

Second, the most common complaint is exactly what you are experiencing with words that begin with the letter "S." This is because as the teeth move and various interocclusal space forms, air passes  more easily with the tongue position necessary for a "S" sound, which usually results in hissing.

Third, this can improve as the tongue adjusts and teeth move into alignment.

Finally, you should always make your dentist aware so that he can track your progress to be sure that your Invisalign case is "on course."

The more you speak with it the fester you will get used to them. Try reading out load with them at home. Unfortunately there is no rule how fast you can get used to things  it varies for everyone.