A person's soul song is a song that they've never heard before, but when it comes on the radio (or is played in some other manner) they immediately know all the words.
That one song that resonates with you on a deeper level than any other song. Soul songs are “nostalgic overloads”, even though you may never have heard it before.
“I only found my soul song last April but it feels as though I’ve been listening to it my whole life”
“I found my soul song yesterday but it’s not even part of my music taste”
A soulmate song is a song that you are connected to. It is a song that you stumble upon and never forget. It is a song you could listen to on repeat for the rest of your life and it's the song you could marry. (there is only one soulmate songs, no matter how many songs you love, there is always one soulmate song)
"Dude please listen to this song, i think it's my soulmate song..."
"Then it must be good."