Anytime I transfer recordings from audio tape to my computer, I have a cable (that has plugs being the same size as those found on the end of a typical set of headphones, usually 1/8" or 3.5mm) and hook it up to the line input jack of the sound card located on my PC. The software I typically use is dBpowerAMP's Auxiliary Input, but there's other good pieces of audio encoding software out there. When it comes to MP3s themselves, I make sure to use 128 Kbps 44.1 kHz, which is standard CD quality. I've been doing this sort of thing on and off for many years.