Is there a list of single mixes that have never been officially released?
If you mean abandoned single mixes, I can't think of anything but "single mix" of Johnny B. Goode (from Concert) on a copyright comp
Keep an Eye on Summer.
If you mean mixes found nowhere else but on singles, I can provide a list according to a reference book I have:
Cuddle Up (B-side to
You Need a Mess of Help to Stand Alone) - more prominent lead vocals compared to the version on the original LP
Susie Cincinatti (B-side to
Add Some Music to Your Day, as well as
Child of Winter) - exhaust note sound effect at the beginning
Hey Little Tomboy (B-side to
Peggy Sue, US issue) - fadeout is 14 seconds longer.
Here Comes the Night (12-inch single) - has a longer intro but clocks in ten seconds earlier than the album ver.
Livin' with a Heartache (single edit, US issue) - clocks in at 3:05, making it a minute shorter.
Rock 'n' Roll To The Rescue - 12-inch single has a "Beach Party Mix" that's three minutes longer. Also, there is a us-exclusive 12-inch single including "Parcadella Mix" with pretty much only vocals and percussions, and instrumental version
Lady Liberty (B-side to
California Dreamin') - Lady Lynda with different lyrics, with Carl on lead vocal partially. Also found on CD single of Still Cruisin'.
Wipe Out - There are a 12-inch single including "Wave 1 Version", a version with DJ scratching, and "Wave 2 Version", a house mix.
Don't Worry Baby (w/ The Everly Brothers) - different from the mix of Everly Brothers album
Some Hearts, with more prominent harmony by The Beach Boys.
Kokomo (Spanish Ver, B-side to
California Dreamin' reissue) - With Spanish Lyrics, and has Brian on chorus.
Somewhere Near Japan (CD single promo mix) - With drum intro, and vocals are more prominent
Problem Child a) Cassette Single - w/ instrumental version
b) promo CD single (RCA/2672-2RDJ) - without children chorus
Forever (w/ John Stamos, CD single) - has two additional mixes - AC mix (the one on Summer in Paradise uk Ver.), CHR Mix
Fun, Fun, Fun (w/ The Status Quo) - has a extended fade version which is a minute longer
Friends (B-side to
Don't Fight the Sea vinyl single by Al Jardine) - an a capella mix
That's Why God Made The Radio - a instrumental mix