I'm making a visualization app, where the user can control instrumentals in an orchestra - volume of strings, horns, etc - similar to how an orchestra conductor would do it...
I am looking for sample audio files to use in this app.
Also - Is there a way to convert an mp3 or other files into multitrack?
you can go for things like "Soundfonts" and apply them on "midi" files but there is no other format for getting pre-recorded "audio" instrument tracks separately. I think Midi files could be enough to fit your needs.
if you choose "midi" format there are many many websites, just Google it. some examples:
about converting audio files to multitracks I should say no. I saw some approaches for converting audio files to "midi" but they are not precise enough and they will not carry the real sounds but just notes. (refer to http://en.wikipedia.org/wiki/MIDI to learn more about midi files)
If you're just taking existing songs (mp3's for example) you could sort of have each "section" of the orchestra segregated by their main frequency bands and just increase/decrease those frequencies. wouldn't be great. Ideally you'd have multitracks of each section. There's a nice resource of multitrack audio at cambridge-mt.com, but not a lot of orchestral stuff. Here is one song that might serve as a good starting point.
Just to expand on the existing answer and make it a bit more clear, multitrack audio doesn't really work that was (sorry). Once a file is mixed down to stereo for distribution, there is no way to split it back into it's component tracks.
The only real option (aside from a midi arrangement like other mention) is to get the "session files", which is all of the original multitrack recordings along with the program data from the application used to record them.
There are some examples of this available, but you probably won't have a great selection to choose from, but you'll have something to build a prototype from at least. Later you might consider commissioning your own recordings to get the session files or asking someone to use theirs.
One example (I haven't used these, so I can't vouch for quality): http://www.cambridge-mt.com/ms-mtk.htm
An analogy that might help with the concept:
If you "flatten" a file in photoshop and then export to a JPEG, you won't be able to get split the JPEG back into the different layers used to make it.
More abstractly: When cooking a meal, if you mix various spices and ingredients to make a sauce, you can't really get those components back after the fact.
You should check out this software https://www.orchplaymusic.com/
From what I've heard the instruments are midi, yet the virtual voices sounds great
