If you've burnt them to CD before playing on your stereo, then it could be the quality of the cd-r media. The old cheapo brands are not worth the wasted time. The worst come from China & Taiwan. Some don't start to distort until after about 6 to 12 months. Get a decent brand of cd media & have it burn well & last for many years. Also burn your media at the minimum speed! Not at 50 speed!

What softw are you using to burn with? Some mp3 softw can allow you to maximise/normalise or add volume to the audio tracks. Should be careful in that regard. I have softw that can normalise to my desired % of the maximum safe level.