Interpreting analog sound signals from line-in

[Arokh]

Hi,
I have an old amplifier which connects to a tuner and a tapeplayer,
which are connected though three chinch cables (2xaudio 1xcommunication).

Now instead of connecting the com-cable to the tapeplayer,
I connected it to a loudspeaker and it was as I have hoped:
As soon as I pressed a button on the amp remote,
I could hear a signal coming from the loudspeaker.

Now the next step of the plan is to connect it to my soundcards line-in port,
and then interpret the signal.

The problem:
I have next to no knowledge about audio processing.
I guess Id have to extract the frequency and the volume level,
so I can differenciate between the different buttons pressed or holded.

Can anyone point me into the right direction where to start,
so I can accomplish this?

If you want I can attach a sound sample of different signals.

I tried some different source code samples, but Ive yet to find one that works:
http://www.codeproject.com/KB/audio-video/cswavrec.aspx
In this one I always get a nullpointer exception.
I already tried the suggestions in the comments but to no avail.