If you have recently added to your mp3 or jpg library and want to backup just new additions, here is the command to create a tar archive of files modified in the last 2 days.
find . -mtime -2 -print | sed -r "s/([^a-zA-Z0-9])/\\\\\1/g" | xargs tar -rvf new.tar
The find command finds the files, the sed command escapes non-ascii characters, the xarg command fires up tar commands which append to the tar archive called 'new.tar'. The find option '-mtime -2' means files that have been modified in the last day days.