For rehearsals, a lower audio quality (i.e. lower bitrate), might be acceptable, with the benefit of having smaller ZIP packages. Here's how to convert existing MP3 files in-place, on Ubuntu Linux.

1. Un-ZIP the package.
2. Run the following script (you might want to adjust the bitrate from 128 bit/second to another value)

   #! /bin/bash
   # Transcodes all audio files in a folder
   # This script works through all MP3 files, and transcodes them in place (A temporary file is required, as ffmpeg does not natively support in-place editing).
   for file in *.mp3; do
   [ -e "$file" ] || continue              # Check if the file exists
   temp_file="${file%.mp3}_temp.mp3"      # Create a temporary filename
   ffmpeg -i "$file" -b:a 128k -y "$temp_file" && mv "$temp_file" "$file"  # Transcode (use your desired bitrate here) and rename
   done

3. Re-ZIP the package (use no compression as zipping MP3 encoded content is inefficient)