Visit Acoustica.com and check out their software program called Spin It Again. Download a free trial version, which will allow you to record from any audio source, like cassette tapes, records or mini-discs (basically anything with an audio output).
Visit NCH.com and peruse their downloadable program called Golden Records. Click the "Try It Now - Free" button to begin the installation process of their trial version, after which you'll be able to record from any audio source and convert to MP3 files.
Point your web browser to FreeDownloadCenter.com and peruse their selection of freeware and shareware programs that convert audio inputs to your computer into digital audio files like MP3s. Choose a program compatible with your computer system (Mac or PC), and follow all directions.