press the Save button to write the .xseq file and .xml file.
delete the .xseq file -- important if you are shrinking the sequence time.
create a music file of the length you want -- or edit an existing file to the desired length. it can be dead silence if you choose, you don't have to listen to it.

copy the music file to your xlights working directory.
now, open a sequence again -- but this time select the musical sequence option -- it is the only option that allows you to pick an xml file as input. pick the music file you created, then select the *.xml file that you created originally. press Save.
the new sequence should be the desired length -- defined by the music file, but pulling in the effects and timing defined the the first sequence file.