For some reason, SM doesn't write bg changes to DWIs.  You have
to add them yourself after saving the SM.  Here's how to do it
for an AVI:

Immediately after the #CHANGEBPM instruction, paste the following
code:

#BACKGROUND:
 M:MOVIE:fr018-bg.avi STARTAT:0.0 LAYER:0;
 SCRIPT:M
#END;

You'll have to make two changes to the code:

1. Change the name after M:MOVIE: from fr018-bg.avi to whatever
   your AVI file is named.
2. For each beat before the movie is scheduled to start, add two
   dots before the M.  That is, one dot for each eighth note.

A full explanation of DWI background scripts can be found in the
DWI manual.

