I have found a workaround!
I "converted" the wav file to a new wav file with the same properties using Blaze Media Pro 6.1 (15-day trial). After that it was possible to open the file i wavepad without any problem.
The strange thing was that wavepad had no problem opening a short recording (4sec) originated from the same MP3 player in the same format without any "conversion".
Still confused but al lot more happier!