So, in case you didn't know, I am part of a WWE Theme Finding group on YouTube and, WWE hasn't been releasing theme songs so much. Like, they just haven't. I don't know if it's because someone has control on music or not. But, anyway, here are a list of themes not released.
About To Be Broken (Brock Lesnar, used in 2002) [Remade several times, but the versions Brock used remain unreleased]
Project Gothic (Christian, used in 1999) [Remade, but original is unreleased]
Al Snow Heel Theme (Al Snow, used in 1999-2000)
Corporate Player (Triple H, used in 1999)
Chief Morley (Chief Morley used in 2003)
3 Minute Warning (3 Minute Warning used in 2002)
It's Too Late (Drew McIntyre used in 2009)
End of Days V1, V2, V3, V4, (The Corre, used in 2011)
I'd really like to see the Al Snow one surface, it was used for quite a while relative to a lot of missing theme music.
If these were done internally, though, they'd rely on either WWE publishing them (doesn't seem like a priority) or somebody leaking them (WWE seem good at avoiding this).