Sometimes the mp3 files will show up but will not play, They just get skipped on to the next song, or they just do not appear on the RNS-E. This is because of the ID3 tags.
The RNS-E only recognizes ID3 version 1. Many programs use ID3v2 and higher.
You can use a program similar to MP3 Repair Tool by Dr. Tag and remove the tags before the first frame and after the last frame:
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It will copy the files and rename them with a *******_repaired.mp3. Delete the orig files and rename these new ones back to the orig file name. A helpfull program is MP3 repair tool, available in the download section.
Make sure to remove before first and after last frames both.
The ID3 tags will be gone from the files. The names still show on RNS as it comes from the file name.