Programming :: SED Assistance: Add Character In The Middle Of A String?

I have a folder of MP3s, where some have <artist>- <song> and others have <artist> - <song>. I wanted to make all of them say <artist> - <song>

The command that works for me was

for i in *[a-Z]-*; do mv "$i" "`echo $i | sed 's/-/ -/g'`"; done

but if I -hadn't- used ls with my parameters to filter out the valid results, how would I have done this with sed? I got close with:

ls | sed -e 's/(.*)[a-Z]-(.*)/1 - 2/'

but with the above I lose the [a-Z] character that I used to match with (and I need to keep that).