I am trying to add a mp3 music file to a slideshow in lightroom 5 and it won't let me. Says the file is not a valid file.
Could you please confirm which operating system you are working on?
Also, I would suggest you convert the Mp3 files to M4A format and then try and add them in Lightroom.
You may try this link to convert the files or try any other third party application. http://audio.online-convert.com/convert-to-m4a
Please let us know if it helps.
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