Thanks C_Major.
I recreated the MXF files with MediaCoder, in "Copy video/Copy audio" mode with only the needed audio track selected, so that I ended up with MXF files with a single audio track..
But if VP was able to open all the audio tracks, which ARE in the source file(s) and which it can open the first track, this would be even better...
Since it can open one audio track and it seems it can manage several audio tracks in a sequence (but I did not find out how to add another audio track yet), the functionalities seem to be already there, it would be much a matter of ergonomics that adding a new functionality per se.
Thanks anyway