I have a doctorate in anal acoustics. While there may be some slight deepening of pitch the more you stretch your anus, almost all of the sound variation comes from other factors such as sphincter squeezing, hair around the butthole, positioning, and even the composition of your butt cheeks.

Might I suggest anal kegels so that you can squeeze those farts out at a higher frequency. This should throw off all butt the most highly trained experts like myself. And we aren’t a judgmental lot.