Hum. You cannot hum without a long out breath, so the technique makes you do it right whether you are concentrating or not.

What the research says

In a randomised controlled trial, a single session of slow breathing biofeedback reduced performance anxiety in musicians, which shows that one stretch of controlled breathing can change how a stressful moment feels. Humming has not been tested against silent slow breathing, so the reason to prefer it is practical rather than evidential: it is the version you cannot do wrong.

Strength of evidence: one line of research. A single study, or a theoretical account.