The microphone port is not a good choice for your signals, as 30kHz to 50kHz is outside the range of human hearing, and the microphone ports are meant for signals well under 30kHz. Some sound cards, however, will support these high frequency ranges. These cards are marketed with having 96kHz and 192kHz sample rates. You might be able to get enough information about your signal to do processing on it with a 192ksps sound card, but it may also prove troublesome depending on the input filtering in the sound card.
Your signals really need 250kHz sampling rate or higher to capture enough information for good processing. This isn't difficult to achieve using hobby electronics, but it's not for beginners either. If you're interested in building something yourself you might want to look toward USB oscilloscopes or the Amateur Radio community with their software defined radios (SDR). There are projects available that have the bandwidth required for your project.
If you're really mostly just interested in actually getting the signal into the computer so you can tinker with it in software, though, I really suggest you go with an off-the-shelf analog to digital converter. Building, debugging, and evaluating one would soak up a lot of time you'd probably rather spend on the actual processing.