I want to merge splited audio files in sub folder with Sox. I have this basic script

#!/bin/bash

# Example: sox_merge_subfolder.sh input_dir/ output_dir/

# Soure directory with subfolder that contains splitted mp3
input_dir=$1

# Set the directory you want for the merged mp3s
output_dir=$2

# make sure the output directory exists (create it if not)
mkdir -p "$output_dir"

find "$input_dir" -type d -print0 | while read -d $'\0' file
do
  echo "Processing..."
  cd "$file"
  output_file="$output_dir/${PWD##*/} - Track only.mp3"
  echo "  Output: $output_file"
  sox --show-progress *.mp3 "$output_file"
done

It is working but I would like to switch to a command that use only folders that contains mp3. To avoid error like sox FAIL formats: can't open input file '*.mp3': No such file or directory

I have this command that work find . -maxdepth 2 -name "*.mp3" -exec dirname {} \; | uniq. But the path are relative and I fail to include it into my existing script.