Start by saying, "Um-hum" with closed lips in a natural manner. Sometimes, "mm-mm" (negative version) works better. Or try saying, "Mmmm, mmmmm, good!" in Southern-drawl fashion. Somewhere in that bunch of hmmmmms, you may find your optimal speaking pitch. Another good way to locate it, is sighing through closed lips on a comfortable descending mmmmm. Start a few tones higher than your normal speaking pitch. If your normal habitual speaking pitch is close to your optimal pitch, or IS your optimal pitch, you may find this easy. Your optimal pitch will be the one that feels the most buzzy behind your nose and lips.

Hint: Average male OSP is from 130 Hz down to about 103 Hz (or an octave below Middle C on the piano keyboard to about G# below that C). Female OSP is usually between 233 Hz down to about 196 Hz (or between A# and G, below Middle C).

There's a lot more to this. But you might have some fun and discover something important about your speaking pitch along the way!