The sound is somewhat deteriorated? How do I fix it?

I have an 8 hour class on Monday I want to record. I am having trouble getting the true sound on a recording. I record a video on Udemy.com but the sound on the recording is somewhat deteriorated. How do I fix it? I was able to use the Shift-Control -5 to record but the sound is not the same as the original video. I there a way to get the true sound on a recording?