solo, so low

So low or so low ( two words ) means very, or to the extent specified ( so ) near the ground or surface, or small in measure or number ( low )--
The temperature is so low that the pond is frozen over.
The table is so low that we'll have to sit on the floor to eat.
So low may also describe a very deep ( low ) voice--
Her voice is so low; people are surprised when they meet her and see how small she is.

Solo or solo ( one word ) means a musical  number or act written for or performed by one person--
She does a wonderful solo in the second act.
He wants to sing a solo, but the director won't let him unless he promises to practice.

Solo may also mean alone, in a general sense, or without a companion or copilot--
She has been solo since her husband died three years ago.
This is his first solo flight, and he's nervous.

Now that you know that, you can say--
"He was asked to sing a solo, but the pay was so low that he refused."