You can use a shell command like this:
for f in *.txt;do say -f "$f" -o "$aif";done Random English voice:
IFS=$'\n';a=($(say -v\?|sed -E $'s/ /\t/'|awk -F$'\t' '$2~/^en_/'));for f in *.txt;do say -v "$))]}" -f "$f" -o "$aif";done
Random voice from a list:
IFS=, read -a a<<<'Daniel,Fiona,Moira,Emily,Serena,Tessa';for f in *.txt;do say -v "$))]}" -f "$f" -o "$aif";done
You can use ffmpeg to convert the files to mp3:
for f in *.aif;do ffmpeg -i "$f" -aq 2 "$mp3";done -aq 2 corresponds to -V2 in lame. You can install ffmpeg with brew install ffmpeg after installing Homebrew.