Many folks use Audacity to transfer their old music cassettes to a digital format so they can preserve them for posterity or play them on their mp3 playback device. The reason you don't hear the same amount of hiss on your old music cassette tapes may very well be because they were recorded using a combination of noise reduction (read hiss reduction) techniques including Dolby NR.ДЄudacity is a product of the modern digital age where new technology has made cassette tapes basically obsolete. From the Wikipedia Article on 'Dolby Noise Reduction System':ДЄll the Dolby variants work by companding, or compressing the dynamic range of the sound during recording and expanding it during playback. These settings will only work on tapes that were recorded using the corresponding type of Dolby Noise Reduction (NR). Many cassette tape machines meant to play music cassettes - will have either Dolby B or Dolby C or both settings as one of the controls available. It's a rather sophisticated technology that must be used during the recording process and played back on a machine with the proper decoder. Thanks to Dolby, the cassette tape became the most popular medium for car stereos until the CD player was released. In the 1960's Dolby Laboratories invented a complex system for reducing this hiss which allowed the cassette tape medium to render tolerable (if not quite hi fidelity) reproductions of music. Tape hiss is the high frequency noise present on analogue magnetic tape recordings caused by the size of the magnetic particles used to make the tape. Here is a quote from Wikipedia article on 'Tape Hiss':

Unfortunately one of the biggest problems with cassette tapes using the magnetic tape format was their propensity to produce prominent hiss. Magnetic tape was one of the first formats that allowed folks to easily record speech or music at home.

How to remove hiss in audacity