escalate, intensify, step_up
Meaning: increase in extent or intensity; "The Allies escalated the bombing"
intensify, compound, heighten, deepen
Meaning: make more intense, stronger, or more marked; "The efforts were intensified", "Her rudeness intensified his dislike for her"; "Pot smokers claim it heightens their awareness"; "This event only deepened my convictions"
Meaning: become more intense; "The debate intensified"; "His dislike for raw fish only deepened in Japan"
There was a sound as of the cooing sob of doves, which seemed to multiply and intensify with each second.