Konami music at least, I haven't seen any other companies such as SquareEnix or Capcom.(Though there is the Mother 3 soundtrack and some Final Fantasy soundtrack as well)
Interesting, I hadn't noticed it till today.
I was just searching for Contra on iTunes and it gave me results of Konami Kukeiha Club.
I noticed that insiden that artist were the original tunes
of Castlevania, Contra and Silent Hill 1-3; each with more than 20 songs, each album only for 10 dollars. Quite a deal.
Also, Michiru Yamane is there, with both SOTN and CoD soundtracks each one also for 10 dollars.
Last but not least, there is also available the CV:DXC soundtrack with 56 songs for the whopping price of 10 dollars as well.
I wonder if other VGM composers are in iTunes as well.
EDIT:
Oh, and both the Classic Contra and Castlevania albums have the original songs as well as some arranged.
This also applies to the DXC album.