You can certainly do this with a soundcard.
Plug the line-in into a scanner audio jack, install the software, and watch the packets fly.
I presume you are using windows, in which case I have had luck with FlexNet32 and the SoundModem drivers.
For an overview, see http://pages.sbcglobal.net/kb5iwt/flex32/flx_32.htm
You can ignore anything about PTT as you won't be keying a transmitter. No need to build any circuits.
Reply if you need help!