This is not about a situation where the condition of the teeth cause physical discomfort to the individual nor is it a situation where they are wearing braces or retainers.
Do misaligned or missing teeth affect singing negatively?
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Only as far as the front teeth - top in particular - will cause some sibilant type sounds to come out strangely, but I guess that when that problem is worked on enough, it can be overcome.
Singing is less definitely affected than reed instrument playing. Try finding a photograph of Freddy Mercury showing his teeth such as this.
On my top row of teeth, my lateral incisors: (https://en.wikipedia.org/wiki/File:Teeth_diagram.png)
are higher than they should be. Additionally, I have quite a large overbite.
I sang small vocal jazz ensemble in high school and in college and these issues have never negatively affected me in any way that I have noticed. Incidentally, I also played baritone saxophone in high school and in college, and never noticed a problem there, either.