To convert cassettes to CDs for free, start by downloading a recording program such as Audacity and connecting an audio cable from a cassette player to the computer. High-grade tape decks produce a better sound, but any tape player with an line-out or headphone jack works.
Computers store sound as digital files, while cassettes are an analog medium. Most sound cards have a line-in or microphone jack that takes an analog signal. While high-end sound cards can digitize analog sound at a higher quality level than typical cards, the quality of the cassette player makes a bigger difference.
Audacity is a free and popular sound recording program. After attaching the cassette player to the computer with a 3.5 millimeter cable or RCA cables, record the tape in Audacity or another program by beginning to record with the program and playing the cassette. If the cassette has multiple tracks, use Audacity to create different files for the tracks.
The Free Lossless Audio Codec audio format stores audio content without degrading quality, but high-bitrate MP3 files preserve the quality well as long as the file isn't recompressed to a different format later. Audacity also has tools to increase the volume of the recording if necessary. After saving the digital files, use a program to burn an audio CD. Another option is to copy the digitized files to a compatible media player.