FFmpeg

Convert all WAV (or other format) files to MP3:(Edit)

Simple, easy, can be long to execute if library is large[1].

cd /volume1/music/
find -name "*.flac" -exec ffmpeg -i {} -acodec libmp3lame -ab 128k {}.mp3 \;

References(Edit)

  1. Explained on this thread: https://forum.synology.com/enu/viewtopic.php?t=53671