ok, good news: i tried to play a song which was bugging with another player, and it worked. quite fine. even better (but strange): the first time i had this problem, i tried to play with Movie Player the song that was bugging on Rythmbox (the one i use by default). it didn't work. this time, after trying with Audacity, i tried on Movie Player. no problem at all.
finally, it seems to be more a Rythmbox bug (eventually because of ntfs??) than a data corruption. but quite strange then that it always bugs on the same spot.
has anybody ever heard of that?