Best answer by djraveinpeace
Thanks for visiting the help forums :)
.cda is not actually an audio file format, its more a kind of link/index file on an audio cd, but you can, in certain circumstances, be forgiven for thinking it is an audio file, the below will give you a bit more info on this:
I would guess, and it is a guess, itunes system maybe recognises the .cda file and "knows" to grab the actual audio...again, a guess there, not an itunes user myself.
You would need to convert the tracks to .wav, .mp3 or one of the other supported audio formats that Soundcloud's systems are able to accept.
Hope this helps.