Data on a cassette

I'm looking at a project that involves taking information and storing into a cassette. The problem is, I am thinking about ways to put the data on there, and all of them rely on some way of making information into a sound or something, like maybe convert it to ascii and then do pulses of certain sounds at certain times, but thats not the point right now. The problem I am having is reading in sound and doing something with it, I could easily convert data into some form of wav format, like the letter A could be a long 44Khz tone or something stupid like that...but how do I read it back into a computer? I thought about raw format and using DTMF tones, but I want some room to play with, so if they are up or down and slightly different then the pure DTMF tone, then its cool, so I can look for things close to them but how do I look for that in a file? I would like to have a file, maybe a wav to raw, and open it up in said program and see that there are 2 pulses of some tone or something like that.

I understand that and all, but still, I am working on a project that interacts with a microcontroller, and can use possible telephone as a transport of data. The telephone thing isn't the purpose, it's mainly to move between the microcontroller/robot to my computer, I know there are methods like RAM and such, but this is something I want to do.
Maybe a way to draw a graph from an audio file of sounds or such, any method I can play with.