I've deleted every music on my n900 (.sounds).
On my PC I've edited the ID3 Tags..
Then I've copied every music back to my n900.
Now the edited things won't show up. Also deleted folders still show up.
How can I reset the mediaplayer to search everything again?

It should eventually sort itself out but if you want to force it to immediately start fresh close down media player, open an x terminal and type in the bit between the quotes "tracker-processes -r".

Open up media player again and wait patiently. It seems to take 30 seconds or so before the "retrieving information on media files" message pops up. Once that message goes away everything should be done.