What is a modifier?
A modifier is a word or expression that makes the meaning of a verb more specific. There it is adverbial.
'He sang whenever he showered.' ( 'sang' is made more specific by adding an adverbial clause of time.)
'He sang with a sob in his voice. 'sang' adds the phrase 'with a sob in his voice' to indicate how he sang.
'He sang in Paris. ' ( Phrase 'in Paris' indicates where he sang.)
'He sang long and raucously' ( Two separate adverbs - 'long' and 'raucously' - suggest the duration and the manner of his singing.