Again, "iTunes" means an application itself. It gets confusing when it is used to mean anything to do with Apple accounts and music and movies and half a dozes other things that people use it for. If by "iTunes" you mean audio files then please say audio files so I know exactly to what you are referring.
If you're worried about the app itself then it will be reinstalled as long as you reinstall Yosemite. If your means of "resetting" involves erasing the computer then anything else on the computer will be gone. Frankly I don't think any time in the past 30 years have I seen anybody mention re-installing a Mac OS version on top of the same version already on the computer. Windows, yes.
As mentioned earlier, drives are cheap. I don't know what size you have in that 2009 model but you could probably even pick up a flash drive that would hold the stuff. Not that I really trust flash drives for true backup purposes but I have used a pair for temporary use. If you don't have a backup for what you wish to do now then you really are living life on the edge as far as those files that seems to be using for professional purposes.
If all you have on this computer are audio files then you could just make a backup of those.