She must be imagining the "taunting laughter" growing "louder". In canon, the Slytherin girls were standing behind Snape and mocking her by silent gesture (which he, of course, couldn't see, not having eyes in the back of his head...)

Poor Hermione. I know Snape is a hero and all, but I really do hate him sometimes...

It is very well written, and how would Hermione deal with her insecurities is an interesting topic. Hope you write more soon :)