This is the mantra of the breath; if you observe your breathing you'll hear "so-hum" as air moves in and out of your lungs. As you inhale, the sound of the vibration is "so". And as you exhale, the sound becomes "hum". If you want, you can experiment with this. Inhale deeply, close your eyes and your mouth, and exhale forcefully through your nose. If you concentrate, you'll hear the "hum" sound quite clearly.
