Here is the original item:
One day in class, Shantrell rested her head against the
wall (A)
and discovered
(B) that she could hear
the classical music that Professor Nguyen played in his office on the other
side. (C)
The music promptly put her to sleep.
You wanted to do this:
One day in class, Shantrell rested her head against the wall and
discovered,
that she could hear the classical music that Professor Nguyen played
in his office on the other side. The music promptly put her to sleep.
A comma after discovered is an error. That she could hear the classical music is a subordinate clause. Generally, you use no punctuation to connect a subordinate clause to the main clause in front.
You might want to review the rules.