use opus since it yields better audio quality

scripts/transcode_opus.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# evaluate options through given arguments
+file_suffix=
+map=
+while [[ $# > 0 ]]; do
+   case $1 in
+      -t | --type) file_suffix=$2; shift 2;;
+      -map       ) map='-map 0:0'; shift;;
+      *          )                 break ;; # Anything else stops command line processing.
+   esac
+done
+
+# ensure existence of destination folder
+dir="$(pwd)"
+dest=${dir%/*}/${dir##*/}_ogg
+if [[ !(-d $dest) ]]; then
+   mkdir $dest
+fi
+
+for file in ./*.$file_suffix; do
+   # `-vbr on -application audio` should be default, but let's be sure
+   echo "ffmpeg -i '$file' -c:a libopus -b:a 64k -vbr on -application audio $map '$dest/${file%$file_suffix}ogg'";
+   ffmpeg -i "$file" -c:a libopus -b:a 64k -vbr on -application audio $map "$dest/${file%$file_suffix}ogg";
+done