I tried to use both:
But neither one worked. Can you recommend a tool or an alternative approach to decode a Morse message saved as an Mp3 file?
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I'm not aware of a tool which will take an MP3 file as an input for CW decoding. You should be able to set up either of the software you tried to use a line in, and connect your computer speaker to the line in jack, then play the mp3 using another program while running either of the above decoders. There are ways to loop the sound through in software, but running a simple cable is easy to do and explain, so try that first.
Can you recommend a tool or an alternative approach to decode a Morse message saved as an Mp3 file?
The old fashioned alternative approach is always available, using your ears.
The advantage of having a recorded (MP3) CW QSO is that you can pause, rewind, listen again. Print a chart of Morse code, listen, transcribe. You may even learn Morse in the process, and be able to listen to "live" CW.
If you have an Apple Mac personal computer running a recent version of OS X, you can play the mp3 files using iTunes, or even the Safari web browser, out any built-in or external speaker, and run the HotPaw Morse Decoder app, available as a commercial product from the Mac App Store. If the volume is turned up enough, the background noise and recorded QRM are low enough, and the Morse Code is stable and well formed enough, the Morse Decoder app should be able to decode it into text.
Disclaimer: The Mac Morse Decoder app is a commercial product that I developed. An iOS version is also available. I used the above method, with mp3 recordings of actual over-the-air CW QSOs, to do regression testing and QA of the app. Hopefully that allows this answer to be suitably on-topic.
If you are using Windows you could probably use the commercial CW Skimmer software in combination with a so called 'virtual audio cable'. A free (as in beer) virtual audio cable driver is VB-Cable. This installs as a virtual audio card in your system, which should make it possible to redirect the output of your MP3 player to CW Skimmer.
Of course, as suggested above, you could also use a physical cable to loop the signal from the output of your soundcard back into the computer. There are two drawbacks with this method though:
Item number 2 above will not likely be a problem, unless you are listening to very weak morse signals. A shielded loop cable (made as short as possible) can be beneficial, but the noise from the A/D and D/A converters can only be remedied by use of a better (and likely more expensive) soundcard.
What SNR you need is ultimately determined by the decoder in CW Skimmer and the 'quality' and signalling speed of the received morse code.
Unfortunately I don't know of any open source software to solve your needs.
You can download many free applications to convert a digital sound of morse or any other digital mode into text. My favorite is the last free version before it was sold, of Ham Radio Deluxe, and use the Digital Master 780 application inside the main program.
Install it, open DM-780 and set the Program Options to read the sound from almost any source your soundcard can hear, such as Line In to get the sound from the MP3 file. You can also open a window to one of the SDR websites, tune the digital radio in to a good frequency with CW or one of the digital modes come in and then set the Program Options to read from Line In, and decode to your hearts content.
I have used an app called Morse Code Reader.
It works well for me!