You didn't give us much detail about the circumstances.
Buffering while watching a YouTube clip I would initially suspect it was to do with your internet connection being too slow.
If the buffering continues after you've disconnected from the internet completely (unplug the cable or switch off the wireless link) then we can conclude it's something amiss inside the PC, such as: -
1) Some software is running in the background (whether you wanted it to or not) and using up processor power to the detriment of processing the music CD (likewise for mp3s on the hard disk).
2) A hardware fault - the soundcard, the motherboard, the cable connections, the HDD or CD drive. Mostly stuff that you can't see and report back to a message thread. Speak to your local repair person.