Adams has written a lot about his voice.

He has/had spasmodic dysphonia which means (rougly) his vocal chords would spasm and seize up during normal speech. It was a huge problem for him. One path he took to getting over it was lots of voice lessons to gain greater control over his speech. What ultimately cured him, though, was a surgery that rewired the nerves in his neck.

It's distinctive (a little slower and more resonant than most people, I think) and it may be something he's worked on.

He has hypnosis training. The style of talking slowly is typical for hypnosis.