The audio files are already compressed as you said they are in mp3 format. You say they are on 8 cds. They must be highly compressed to fit that much only 8 cds. A cd has 700 MB of storage. You have 8 of them. Do the math and that equals about 5.6 GB. If that's the case, you should probably get something that is at least 8GB. Go with 16GB if you plan on adding more.
Now if they were on DVDs, that would be another story. Are you positive that they were cds? A regular DVD has about 4.7GB of storage and doing the math, that would be 37.6 GB. Of course, each of these DVDs may not be entirely full.
That aside, if the music were originally ripped at 128 kbps, I believe that would take up 1MB per minute of music. So each hour would take up 60 MB. So, yes approx 16 hours per GB. And if you had 500 hours, that would be 30 GB of music. Then again, you didn't say they were music files. You merely said they were audio files. They could have been done at a much lower bitrate, which I think is done with spoken word.
30 GB or 37.6 GB, that is going into hard drive player territory. However, you could get a 32GB flash based player and expand via SD or microSD cards.
Instead of just guessing, you could put all of that audio onto your computer, find out the resulting size of all of those audio files in your music folder, then take it from there.
I have 500 hours of mp3 files on 8 cds that i want to load onto an mp3 player but I can't figure out what size player I need. I know 1 gig is 16 hours of audio files, but mp3 files are condensed. Right? Someone help please!!!

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