Audacity 2.4.2 
I have been successfully converting old cassette tapes to mp3s and then processing them in Audacity BUT,
the latest cassette recording is not accepted as mp3 (but it most definitely is). I also tried the import and what came out
was some 40mb of a very loud buzz. The msg says it doesn’t recognize the format and I should install FFmpeg. the file plays perfectly
with all my Windows players. What I don’t understand is after over ten cassettes converted this way with no problems, why would it
reject this one?
converting old cassette tapes to mp3s and then processing them in Audacity
As a completely side issue, that’s the not the best idea right there. Audacity doesn’t edit MP3. It converts MP3s to its own super high quality, internal format and edits that. So far, so good. But then it has to make a new MP3 when it’s done. That is what you want, right?
When it makes the new MP3, it doubles the compression and encoding distortion. With all the tape hiss and other distortions, you might not notice, but it’s there.
What you’re supposed to do is File > Export Perfect, uncompressed WAV (Microsoft) 16-bit files and that’s your forever archive. Then make the MP3s from that and you won’t notice the distortion because you will only have one compression pass. WAV files aren’t compressed.
You can get close. In the tape playback, File > Export > MP3 Constant bitrate, 256 or 320 quality. Some really high number. That compression distortion will be really tiny and when you make the lesser quality MP3s for your portable music player while you’re jogging around the neighborhood, you won’t notice it. Note that the original MP3 is your forever archive, not the lower quality stuff for your music player. You can throw those out when you’re done. You can always make more from the higher quality archive MP3.
Do Not step on the original files while you’re exporting the trash versions. That destroys your archive.
You’ve been making all the mistakes, right? Make a low quality MP3, edit that and save the changes over the original MP3 file. That creates a permanent low quality archive with permanent MP3 distortion and no possibility of rescue. Don’t lose those tapes.
Any other word on why one of them is distorted?
Koz
Did you create the MP3 with Audacity (Export as MP3)?
Check the questionable file with [u]MediaInfoOnline[/u]. Copy and paste the results here.
Koz:
I should have stated that I am a novice user. I have only one cassette player (cheap) which actually does the converting. I think I understand what you are
saying but I’ll have to learn how to transmit the cassettes directly to Audacity. I’m glad you brought this up because these are very valuable old tapes and I would love to get better results.
DVDdoug: here’s what I get:
General
Complete name : FV0001.mp3
Format : MPEG Audio
File size : 42.6 MiB
Duration : 46 min 32 s
Overall bit rate mode : Constant
Overall bit rate : 128 kb/s
Audio
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 3
Format settings : Dual mono
Duration : 46 min 32 s
Bit rate mode : Constant
Bit rate : 128 kb/s
Channel(s) : 2 channels
Sampling rate : 44.1 kHz
Frame rate : 38.281 FPS (1152 SPF)
Compression mode : Lossy
Stream size : 42.6 MiB (100%)
I appreciate both of you and Koz’s guidance
The MediaInfo data looks OK so I don’t see any clues there.
I know, I can’t figure it out.
[u]MP3 Diags[/u] can diagnose and repair lots of MP3 problems but it might be easier to re-digitize the cassettes.
If you try MP3 Diags, note that changes are permanent so make a back-up of the MP3.
New: I tried to load an mp3 that has previously been loaded and still got the rejection popup
I looked at the Diag thing but it’s too complicated for me. 
I installed ffmeg and still nothing. When I tried to import it this is what I got.
I don’t understand as I’ve done several transfers with the same machine without this problem,
I am going nuts here. I’ve tried a direct line from the cassette player to the computer using the mic in and the blue mini plugin
in back and every combination of recording and playback and I still get no sound. What am I doing wrong???
…You didn’t say what hardware & software you were using to “successfully” make MP3 files. If you make an MP3 with Audacity you should be able to open it with Audacity. But as Koz says, it’s best to make a WAV file and/or save an Audacity project and make the MP3 as the last step after all processing/editing.
I’ve tried a direct line from the cassette player to the computer using the mic in and the blue mini plugin
in back and every combination of recording and playback and I still get no sound.
That’s a different problem… Is that how you did it before or do you have a cassette player with a USB connection?
Line-out, tape-out, Aux-out, or record-out (typically [u]RCA connectors[/u]) or headphone-out will work into line-in (blue) on a soundcard. (The mic-input is “wrong”.)
If you are using a headphone-output make sure the volume is turned-up and check with headphones to make sure there is a signal.
Make sure you’ve selected line-in as your [u]Recording device[/u]. (Your screenshot shows “Stereo Mix”.)
You can test line-in with [u]Listen To This Device[/u]. That should work without running Audacity or any other application. That’s just for testing… If you can get everything working, turn OFF Listen To This Device and turn-on [u]Software Playthrough[/u] to hear what you’re recording.
With Audacity, the MP3 or WAV is not created until you Export as MP3 or WAV.
I looked at the Diag thing but it’s too complicated for me
I agree! 
There is a TON of information on the MP3 Diags website but I haven’t read it all and I don’t understand it all either…
If you click on All Notes you should get a list of potential problems with your MP3. Then, on the top menu you’ll see hammer icons which are the repair tools. I believe #4 attempts to fix “everything” so try that (after backing-up the original file).
WOW thanks so much. I did try using mini plug from small cassette player to the blue input in the computer but got nothing. I did try the mic input too but same result. I will go thru the other suggestions you have. I really appreciate you hanging in there!
BTW, I have a portable cassette to mp3 converter but when “Koz” first mentioned that I was doubling the compression etc I have been trying to learn to use Audacity as the recording software.