I am new to this website but have been a huge ABBA fan since childhood. Recently I discovered this incredible YouTube channel named "Peter Loner" who creates these amazing tracks of original ABBA songs (among other artists like Aretha Franklin, Karen Carpenter...) and turns them into high quality filtered instrumentals with backing vocals and masters them to sound like original soundtracks or instrumentals/demos.
I highly recommend that you check Peter's channel out and give him some support! Tell him I sent you, haha!!
Here is a list of ABBA Instrumentals & Backing Vocal Tracks he has filtered and created with the links to the videos (so far, Dec. 14 2016):
My Mama Said
Head Over Heels
Ring Ring (Spanish)
Why Did It Have to Be Me
Knowing Me, Knowing You
Our Last Summer
When All Is Said And Done
Take A Chance On Me (Amazing Recording Session RARE!)
Does Your Mother Know
Felicidad (Happy New Year Spanish)
Jag Ar Blott Man
Sitting In The Palmtree
Voulez Vous (Recording Session)'
Waterloo - Honky Tonk Filter
Spanish Songs Medley
You will find many more songs on his channel from Agnetha and Frida as well!!
It's really interesting to hear all that. The mix of My Mama Said is particularly interesting as Agnetha mispronounces "doubt" in the last verse which is hidden behind a more correct pronunciation in the final mix.