Sound quality is poor even though a music CD is played back digitally with my notebook computer.

In spite of the digital playback of a music CD through the built-in speakers, the sound quality (the frequency and balance between treble and bass of the reproduced signals) is poor.

If you place importance on sound quality, you had better use external audio systems.

You are suggested to use external audio systems if you prefer to have better sound quality.

"The digital reproduction of a music CD" stands for the reproduction of the digital data as it is from a CD-ROM drive to Audio Codec (the data of audio signals is reproduced
without degradation through the data transfer).
That is, the data will be reproduced with less influence of the noise coming from the electric circuits of the computer, which does not necessarily mean better quality sound.
"Sound quality" depends on the output/reproduction of certain frequency ranges and certain amplitude levels of analog signals. Whether the sound source is digital or analog is not the determining factor of the sound quality.

The audio function of Windows computers does not have the controlling application that is capable of equalizing frequencies in narrow ranges. If you place importance on sound quality to play back a music CD with your notebook computer, adjust sound to what you like best with the use of external audio systems (amplifier, speakers, headphones, etc.) instead of the built-in speakers.