Answer:
The answer to your question would be that in the sentence the word singing functions as the subject.
Step-by-step explanation:
The subject of the sentence is the person, thing, or idea that is doing or being something. In order to find the subject of the sentence, first you need to identify the verb. Then, you ask the following question: Who or what "verbs/verbed"? (what did improve after he took voice lessons?) The answer to that question is the subject (Evan's singing).