First, understand that MP3 is a low quality file type. It is great for size/space but not a great audio format.
My suggestion is that you record in the cassette as .wav format. This is a higher quality format and would be better for doing any editing or cleaning restoration. Once that is done, and your happy with the results from within the software, you can then export that out as a .mp3 file. It should sound a lot better doing it in that way.
I have my connection from my stereo receiver going into the LINE-IN (blue port) [NOTE: it must be the line in port or your recording will be very distorted. Many people want to use the microphone port, but that is mono (left channel) and uses a gain boost which will cause damaging distortion if played loud through speakers] portion of my sound card. All my other devices (turn-table, cassette, reel-to-reel, compact disc) are connected to the stereo receiver, I just make the proper selection for the device I want to record.
In the software I test that I am receiving the signal. I also always record into .wav format since I want the highest quality recording. Once this is recorded in, I can go through the cleaning and enchancing process, when that is done, I save the project then I export to the format I want.