A problem with the MP3 encoding / decoding algorithms and/or the player's capabilities. An audio CD contains a table of contents, that is read by a CD player and appears as individual small files if you view the disk directory from a computer. The audio data is stored in a reserved section of the CD in a specific data format: 44.1 kHz, 16-bit PCM digital, as defined by the Sony / Philips Red Book. Any CD player can read and play the audio data recorded on the disk. Computers use software that is designed to read or write the audio data according to the Red Book standard. An MP3 file, on the other hand, is a highly-compressed data file. These files can be read, copied, and deleted like any normal data file. A software or hardware MP3 player reads these data files and decompresses them for audio playback. If you place a CD with MP3-format music files on it in a standard Red Book CD player, it cannot read them because they are in the wrong format. To play them on a Red Book CD player you need to convert the MP3 files to 44.1 kHz, 16-bit WAV format and record them using software that can create audio CDs. The audio CD software will convert the WAV files to PCM format, place them in the reserved audio area on the disk, and create a table of contents. A Red Book disk is playable on all CD players. Some CD players, usually DVD players that are CD compatible, can read MP3-format data files from a CD, decode them, and play them back. There are limitations on what files they can play, however. If the file is stored deeply in a subdirectory structure, these players may not be able to find them. Moving the file to a higher-level directory might help. If the MP3 encoding was not done correctly the audio files may not play back. There are a number of MP3 encoding algorithms floating around and not all of them encode and/or decode audio data properly (the LAME encoder is probably the best available). Dedicated MP3 players are usually tolerant of variations between encoders. It is also possible that the problem lies in the firmware in your player and that it cannot reproduce MP3 files that are compressed in a particular way. I would first test the disk on at least one other player that claims to be MP3-compatible. If it plays properly, the problem probably is likely with your own player. If it does not play properly, the problem lies in the disk itself, either because the MP3 encoding algorithm(s) used had irregularities or the file is located too deep in a subdirectory structure. The player must (claim to) be MP3 compatible, otherwise it cannot play MP3 files from the disk. The best solution - the one that provides the greatest flexibility - is to create a mixed audio + data CD. This disk would contain a set of MP3 files in the data area, encoded with a reliable encoding algorithm (e.g., LAME), and PCM audio data in the audio section. I have done this with files that have come in MP3-format and were not available otherwise. I convert the MP3 data to 44.1 kHz, 16-bit WAV files, and create a mixed audio + data using both the WAV files and the MP3 files. The disk can be played just about everywhere, since it is compatible with all Red Book players and the MP3 files can be read using a computer.