Ok I've been trying to send the command before the end of the playing file. I've found a sweet spot, 600ms before the end of the playing file, where I send the playFileNum() command with a little to no clipping sound. It's not perfect but acceptable.
But I loose a part of the playing file. So next thing that I'm gonna try is to edit my wav files for a little longer to be able to send the playFileNum() while the current file is playing and before 600ms of the end. Exemple : if I have a 5000ms file now, I'll will make it 6000ms, so I can send the play new file command at 5000ms. Thus the file will play the initial 5000ms and be cut by the command play function, but clipping/popping sounds should be minimum.
It seems that sending a command to play a new file near the end of the current playing file cause some clipping/popping sounds while changing file...
I'll let you know if it worked out !