I found it in a new version of mplayer:
https://launchpad.net/~rvm/+archive
* the option to change the playback speed without altering the pitch will work (thanks to the new scaletempo filter)

It does not work in maemo mplayer rc1.2. Any body knows where I can get a version with this scaletempo filter ?

EDIT: Here are the scaletempo patches to mplayer rc1.2:
http://sourceforge.net/project/showf...kage_id=279916
Any body kind enough to patch it and make it available for everybody? Thank you in advance!