connect your n8 to your pc via the USB cable. If you are on windows, a new drive (say n:) appears. On linux, a new mounted drive appears (same as OSX). Make sure your n8 is in mass storage mode (either settings->connectivity->USB, or click the icon in the top right on the home screen).
Go into the new drive. There should be lots of folders. Go into the folder named "private". You should see lots of new folders all with wierd hexadecimal names (uses characters 0 to 9, and A, B, C, D, E, F). Go into the one named: 10281e17 and delete the three files ending in ".db". Go back to the private folder again. Go into 101ffca9 and delete the harvester file.
Now start up the music app - it should say something like "searching...."
I'll assume you run windows, so to summarise:
make sure the music app is not running
make sure your n8 is in mass storage mode
connect to your pc
a new drive appears (we'll call it n: for now)
go to n:
go to private
go to 10281e17
delete the 3 files ending with .db
go back a directory to private again
go to 101ffca9
delete the file that starts with harvester (should be the only one in here).
In windows, click on the removable media icon in the taskbar
click safely remove on n:
remove the USB cable when it tells you it is OK
start the music app
it should search for all the music - and won't find the stuff you deleted.
Should be all