Hey,
I acess all my videos and mp3s using Amarok in Slackware 11 in KDE. All mo3s and mpgs works fine. Recently a new mp3 I got is not working correctly. In Amarok it gives a weird crackling sound during vocals. But then I played it in mPlayer and it is crystal clear (well as crystal as 128 kbps can be )

What is this weird problem? I thought both amarok and mplayer use same codecs as in /usr/lib/codecs.
How can I fix this ? How can I make Amarok use the mplayer codecs?

That's the "problem": mplayer probably does use /usr/lib/codecs/, but Amarok itself doesn't "play" anything, it needs an engine to do that, and it depends on the engine which files Amarok will play. I guess the usual configuration is to have Amarok use Xine as it's engine, but gstreamer is possible too; you can check this out at Amarok's configuration. Once you've figured out which engine it uses, you'll have to either

1) point the engine (like Xine) to use the same codecs directory, or
2) install additional codecs package (for example libxine-extracodecs) and hope it fixes the problem.

Mplayer and Xine may use the same plugins (the directory), but not necessarily. It depends on the configuration, so if Mplayer is fine, go ahead and find out which engine Amarok uses, switch it to Xine if needed and make sure Xine uses the same codecs directory than Mplayer does.

yeah both mplayer and xine use /usr/lib/codecs

Also Amarok is configured to use Xine engine. There is no other choice available

Any Ideas what I should try doing now?

And wait it just got weirder.
When I add the "problem" file in Amarok .. I can play in the File Selection dialogue. There it works fine with no problem but when I add it to the playlist then it screws up.