Getting braces is a rite of passage that most of my middle schoolers go through, and it can be a frustrating time for musicians who are just starting to listen to themselves and self-analyze their playing.
Having had braces myself, I can sympathize with the feelings of frustration that come with trying to play flute with a whole bunch of metal in your mouth. Here's my best advice for flutists who are staring down the barrel of a shiny new set of orthodontia:
1.) Buy braces wax! This non-toxic, minty wax is available at most drugstores and grocery stores in the dental aisle, and putting it over your braces will protect the insides of your lips and cheeks from chafing while you play.
2.) Be prepared to adjust your embouchure. The extra bulk between your lower teeth and lip will shift the direction of your air stream, so you might find that you need to roll the headjoint more toward or away from you to get the air stream to hit the edge of the embouchure hole just right. You may also have to place the flute higher or lower on your bottom lip.
3.) Be prepared to sound fuzzy. While you're figuring out your new headjoint-lip relationship, your sound will likely be unfocused since the air stream isn't being aimed properly. This is not a failing on your part, nor are you getting worse at the flute. It's simply a byproduct of your mouth adjusting to a new embouchure.
4.) Be patient! This is a huge transition. There will be days, especially after your orthodontic appointments, when your mouth is simply too sore to make it through a practice session. If this happens, use that day to study scores, listen to recordings, and research the historical significance of the pieces you're studying.
The good news is that once you get used to having braces, your sound will return to normal. It's a temporary challenge, but the results are worth it.