The one-liner by @user7343148 worked really nicely from the command-line, but I had some trouble figuring out a way to make an alias for it and add it to zshrc. So, putting it here just in case someone needs it.

mp3len() {
  mp3info -p '%S\n' *.mp3 | awk '{s+=$1} END {printf "%d:%02d:%02d\n", s/3600, (s%3600)/60, s%60}'
}