Alright, Ed, I suppose I was bemoaning...you are right, however, it can be done without damaging the original(by the way, Brakn, save the original cassette).

Given that, here are my thoughts.

First thing, you'll need to clean up the tape noise. This is important for a couple reasons--first, because a cassette recording is going to have a lot of noise on it, second, because whatever new sounds you record won't have that same noise, and will stick out.

Now, what you want to do is create another track that you're going to ultimately mix with the original in a way that replaces the bad notes with good ones. You'll most likely replace passages, and not just the single notes, because you'll want it to flow more smoothly.

If you're lucky, you'll be able to do what Ed mentioned above, which is to cut and paste the better version of the phrase onto the second track underneath the bad spot, and get rid of the bad notes in the mix.

If you're not lucky, your mom, like many of us, repeated the same mistakes so there is not a clean version of the passage you want. In this case, you'll have to learn the piece (or at least the problem parts) and record them on another track so you can mix them with the original.

I suggest that you look on YouTube for tutorials on editing in what ever program you end up using (Audacity, for instance) watch them, and play around a bit editing simpler things til you start to feel comfortable. It takes the pressure off if you're not learning while you work on something that is important to you.

Warning!! This cutting, pasting, mixing process takes up lots of time, and can get to be addicting. Many of us do it at night, because there's more time then.