It seems tons guitarist can't understand how to use backgraound
music to jam with hard/metal rock stars.
This is explanations:

Many sites, see links here:
http://launch.groups.yahoo.com/group/guitarfx/links
have backgraound mp3+tabs for famous rock music.
But GuitarFX can playback wav file only.
The solution is to convert mp3 into wav.
Old freeware WinAmp 1.4 can do it for free.
It's in File section of this Group.
http://launch.groups.yahoo.com/group/guitarfx/files

Recomended fx-chain:

1)noise gate
2)EQ
3)Distortion
4)EQ
5)Reverb
6)gain
7)Playback