I use a mac retina and want to change the pitch coming out of my mac. Its like if I am listening to music, I can up the pitch (like in garage band) and make it sound like a chipmunk singing. Is there any free apps to do that?
I am not aware of anything that will do this task in real time as it requires some math to do the pitch shift. It is possible to take the alert sounds and process them to a different pitch like you said using an audio editor and then assign custom sounds, but an ongoing pitch shift would take some processing and I do not believe that there is a solution for that currently.
I have experimented with changing sample rates and the system adjusts for that and plays the proper pitch so it is not a simple task.
This is possible with the free app Audacity. That is, it will work if you are editing sound and wish to change the pitch of a recorded sound. I am not aware of any free program that will do this with live sound, on the fly.
Audio Hijack Pro can change the pitch live and in real-time. It takes a bit of fiddling around though, but it works well
So, Garage Band is a free app, and you already mentioned you can do this from Garage Band... so... maybe you need to specify exactly what you are trying to do.
You should try to look into what are called "audio routing" apps, you can find several (some free). These will let you take output from one program (e.g. iTunes) and send it as input into another program that can apply your audio filter.