I have not had much time to tackle the problem but it comes definitely from the Imac. I unearthed and old iMac G5 10.4.11 system from my basement which runs 10.4.11 os ans it plays faithfully the .wav files which are then recognised by the DKC and played perfectly: AH !
So now why the much newer Imac isnt able to do that is a mystery . I ve done eliminating games and found that it is not a software problem (it occurs with all kinds of players), nor a hardware problem as far as I can see ( same result from phone jack out, usb audio out, bluetooth out) ... so Im a bit at lost and the machine not being waranteed anymore Im hesitant to bring it to a service center with such a precise and undectable flaw...
Latest: I found that in the best cases the audio output is recognised by the DKC which outputs it but the sound is clipped like half second of music half second of silence, very regular and it goes on and on with NO midi piano part played though..
In the meantime I upgraded the Imac to the latest Sierra OS and things are not working as it should ... I have to downgrade ... no time for all those exercises LOL
Regards to everyone ; -)
Alain