Sounds good.. getting there
I need some clarification on this.... is it the frequency AMPLITUDE, or the FREQUENCY?
In other words, if you stuck that jack from the device into a oscilloscope, would you get a sine wave? or a straight line?

See.. if I have to decode the frequency, that's a lot more complex. Thankfully not impossible since I already have some free sourcecode to determine the fourier transforms (its like a multi-dimensional map of the frequencies)

Let me try explain it another way: the digitiser in the soundcard works by taking voltage readings, and converting it to a positive or negative number. I then read all those values (called samples).
If you open sound recorder in windows - that green line is the samples I'll see.
If you record yourself singing a note, you'll see that they form a sine wave with negative and positive values

So ... are you going to be providing a sine wave note, with frequency? Or simply an amplitude value on the samples.
Is that line in sound recorder going to be dead straight, or form a wave?

Another problem. If you are giving me amplitudes only, I have to determine how the negative values work. Hopefully you will end up giving me only positive values

For the time being, my amplitude chart is using the average absolute amplitude of the samples, averaged out over each second, and distance between each point of the chart. So if you are giving me a nice sine wave, I don't think it's going to work

When I get some time, I'm going to try convert the sourcecode to Borland C++ 4. I have a free compiler from them. That way it's both legal, and much faster to develop in. The disadvantage is that Borland pisses me off from time to time :