why does this author use single quotation marks?
This is one of the main reasons why even the most accomplished singers have to listen to the opinion of coaches and voice teachers as to ‘how they sound,’ whereas no concert violinist would have to do such a thing.

---
Why does this author put single quotation marks around the words "how they sound"?

Question

why does this author use single quotation marks?
This is one of the main reasons why even the most accomplished singers have to listen to the opinion of coaches and voice teachers as to ‘how they sound,’ whereas no concert violinist would have to do such a thing.

---
Why does this author put single quotation marks around the words "how they sound"?

Answer

In American English, the single quotation marks in an instance such as this would only occur if the entire passage were itself a quotation, in other words surrounded by double quotation marks. To say it another way, in American English, single quotation marks enclose quotations within quotations. It is different in other English-speaking countries.