I found the solution ! Problem is due to maemo's extra repository that provides a special version of libsdl-mixer1.2 (lversion 1.2.6-5+0m+ogg+mp3 when mp-fremantle-generic-pr needs 1.2.6-5+0m). So here follows my workaround :
First, I disabled maemo's extra repository, using the application manager (fapman seems to not manage this operation very well)
Then I removed the libsdl-mixer1.2 package :
apt-get remove libsdl-mixer1.2
Then I reinstalled it :
apt-get remove libsdl-mixer1.2
Then I installed mp-fremantle-generic-pr :
apt-get install mp-fremantle-generic-pr

And it worked ! I'm now on PR1.3