Skipping find and using bash:
shopt -s globstar # enable recursive globbing
for f in **/*.xls{,x} # for files ending in .xls or .xlsx
do
in2csv -n "$f" | # get the sheetnames
xargs -I {} bash -c 'in2csv --sheet "$2" "$1" > "${1%.*}"-"$2".csv' _ "$f" {} # {} will be replaced with the sheetname
done
