#!/bin/bash

find res/sound/ -type f \( -name "*.mp3" -o -name "*.wav"  -o -name "*.ogg" \) -print0 |
while IFS= read -r -d '' file; do
  out="res/mono/${file#res/sound/}"
  out="${out%.*}.ogg"

  mkdir -p "$(dirname "$out")"
  ffmpeg -i "$file" -ac 1 "$out"
done