According to the technical paper on the radio, the mp3 files have to recorded with MP3 ID Tag v1.0 which is a setting you set within the ripping program. Read below:
Notes On Playing MP3 Files:
Supported Medium Formats (File Systems)
The medium formats supported by the radio are ISO 9660 Level 1 and Level 2 and includes the Joliet extension. When reading discs recorded using formats other than ISO 9660 Level 1 and Level 2, the radio may fail to read files properly and may be unable to play the file normally. UDF and Apple HFS formats are not supported.
The radio uses the following limits for file systems:
• Maximum number of directory levels: 15
• Maximum number of files: 255
• Maximum number of folders: 100
• Maximum number of characters in file/folder names:
• Level 1: 12 (including a separator "." and a 3-character extension)
• Level 2: 31 (including a separator "." and a 3-character extension)
Multisession disc formats are supported by the radio. Multisession discs may contain combinations of normal CD audio tracks and computer files (including MP3 files). Discs created with an option such as "keep disc open after writing" are most likely multisession discs. The use of multisession for CD audio or MP3 playback may result in longer disc loading times.
Supported MP3 File Formats
The radio will recognize only files with the *.mp3 extension as MP3 files. Non-MP3 files named with the *.mp3 extension may cause playback problems. The radio is designed to recognize the file as an invalid MP3 and will not play the file.
When using the MP3 encoder to compress audio data to an MP3 file, the bit rate and sampling frequencies in the following table are supported. In addition, variable bit rates (VBR) are also supported. The majority of MP3 files use a 44.1 kHz sampling rate and a 192, 160, 128, 96 or VBR bit rates.