I tried with the sixth rendered files but the problem is still there. This is happen with many files used in "transcode" mode.
I will for the moment use non transcoded file.
I use the last version of UMS and I've got bad synchro sound track (3 or 4 second late)
I tried to set the transcode buffer from 200 to 400 but the problem is still there.
Thank you for support.