Breathing helium or SF6 should not change vocal-fold frequency, only the sound’s timbre.

Evidence

People describe SF6 as making the voice dramatically lower, and indeed the “Darth Vader” effect is more stunning perhaps than the Donald Duck effect. But does either gas really raise or lower the frequency, or even the pitch of the voice? (We will see in chapter 23 that frequency and pitch are not always the same, even for a periodic tone.) Why should it, given the fact that the vocal folds are supposed to do their job at frequencies that they, not the vocal tract, determine? The vocal folds are a high-impedance drive that normally oscillates at low frequencies compared to the tract resonances, independently of them. — Eric J. Heller, Why You Hear What You Hear- An…, 1712

Categorization

• QUESTION: Does inhaling helium actually change perceived pitch of the voice, or only formant frequencies? The author promises the answer in chapter 23.
• CANDY: The Donald Duck vs Darth Vader voice effects from helium and sulfur hexafluoride are perfect casual conversation material.