Magnetic tape was one of the first formats that allowed folks to easily record speech or music at home. Unfortunately one of the biggest problems with cassette tapes using the magnetic tape format was their propensity to produce prominent hiss.
Here is a quote from Wikipedia article on "Tape Hiss":
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.
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. Thanks to Dolby, the cassette tape became the most popular medium for car stereos until the CD player was released. It's a rather sophisticated technology that must be used during the recording process and played back on a machine with the proper decoder. 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. These settings will only work on tapes that were recorded using the corresponding type of Dolby Noise Reduction (NR). From the Wikipedia Article on "Dolby Noise Reduction System":
all the Dolby variants work by companding, or compressing the dynamic range of the sound during recording and expanding it during playback.
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.
Audacity is a product of the modern digital age where new technology has made cassette tapes basically obsolete. 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. Unfortunately, the built in noise reduction effects in Audacity are geared towards reducing the hiss when recording TO Audacity from a cassette and not recording FROM Audacity to a cassette.
You might be able to reduce the hiss some by playing around with the effects in Audacity starting with the noise removal effect and also try equalization and high pass filter. Expand the wave form envelope as much as you can and focus on the higher frequencies. You might try recording the tape hiss back to a digital format so you can open the file with Audacity and see what frequencies the hiss lies in so you will know where to focus your tinkering.
Just be aware than the Dolby system was very complex and sophisticated which is why they were able to patent it and no other company offered a competing technology. Any success you achieve reducing hiss with the audacity controls will also reduce some of the musical fidelity. The developers and ongoing development of technologies such as Audacity, never envisioned producing an audio file for the specific purpose of transferring said file to magnetic tape.
One other thing that I have discovered, is that one of my older stereo digital voice recorders generates noise artifacts when I try to input from my computer into the mic in jack using a TRS connector while one of my newest stereo digital voice recorders does not. So there is a possibility that some of the problem could be in the mic input jack as well. Either way, it might be worth a shot to try playing back your digital recording through the best speakers available and record to your cassette recorder using the built in mic and see if the results have less objectionable noise.