How to make dired-do-async-shell-command wait for all sub processes to finish

Question

I'm trying to run the bash script, ~/bin/xlda, ...

#!/bin/sh
# ~/bin/xlda

old_filename="$1"
new_filename=`echo $old_filename | sed 's/\(.*\.\)flac/\1m4a/'`

~/bin/xld -f alac -o "$new_filename" "$old_filename"

new_md5=`~/bin/xld --stdout "$new_filename" | md5`
old_md5=`~/bin/xld --stdout "$old_filename" | md5`

if [ "$new_md5" == "$old_md5" ]
then
    echo "new file md5 $new_md5"
fi

...on files in a find-name-dired buffer using dired-do-async-shell-command in this function

(defun my-dired-files-to-alac ()
  (interactive)
    (dired-do-async-shell-command "~/bin/xlda" current-prefix-arg
     (dired-get-marked-files t current-prefix-arg)))

The xld processes all finish but the md5 is only echo'd for the first shell script to get there. If I run the shell command that emacs runs in a terminal, i.e. ~/bin/xlda folder/file.flac&~/bin/xlda folder/file2.flac then all the md5s are echo'd as expected.

Ultimately, I will rm the old file if the md5s match. Even if the script is simply...

#!/bin/sh
# ~/bin/xlda

old_filename="$1"
new_filename=`echo $old_filename | sed 's/\(.*\.\)flac/\1m4a/'`

~/bin/xld -f alac -o "$new_filename" "$old_filename" && rm "$old_filename"

...it still fails to delete all but one of the *.flac files.

The problem is that emacs seems to exit all shell sub processes when the first shell sub process to finish finishes.