On what sort of Hi-Fi system are you trying to play the files? After copying them to the USB stick you could first try to play them on your computer (or a different computer) but from the USB stick, this will confirm that the ripping and copying bit was successfull.
If the files from the USB stick are playable on a computer, then I would suspect that your Hi-Fi system doesn't support the specific file type that has been generated. Within the MP3 file format there are several settings that could still cause trouble.