edit: Also, another useful thing is <command> | xargs -n1 -I{} <stuff>, which runs stuff once on every element of the output of command, replacing every occurrence of {} with the element. Then you can do something like "ls *.mp4 | xargs -n1 -I{} mv {} `basename {}`.mp3" .
Reduce, on the other hand, is a pain; a simple > $$.out or similar on the xargs commandline probably won't work, best to wrap it up in a standalone script that does the same thing to guarantee you've got a new pid.
Then you get something like:
$ cat batch.sh
#!/usr/bin/sh
do_something_expensive $1 > tmp_$$.out
$ find -print0 | xargs -n 1 -0 -I{} -P 4 bash ./batch.sh {} && cat tmp*out > batch.out && rm -rf tmp*out
for f in *.wma; do ffmpeg -i "$f" "${f/.wma/.mp3}"; done