This kind of projects would be much easier in the previous version of GarageBand - GarageBand '11, version 6.0.5.
In the new GarageBand the vocal transformer is missing, that could be used to change the pitch of vocals, without making it sound like a monster or a chipmunk.
If you install manage to GarageBand '11, you have the vocal transformer in available in GarageBand 10.0 as well. You will find it in the Smart Controls, if you pick the "Legacy" instrument "GarageBand > Vocals > Deeper Vocals". To use it, change the pitch to lower or raise the voice, and adjust the formant to make it sound natural.
It is easier in GarageBand '11. There you can add a vocal transformer effect to any vocal track and have a larger range of adjustment.
So then how do I install GB 11?
Is your mac new and came with Mavericks, or did it come with system installer disks for an older system? Or with Lion or Mt. Lion and iLife preinstalled?
- If your mac came with installer disks, you may find GarageBand '11 on the second install disk with with the bundled applications. Install it from there (to a subfolder of Applications) and update it from the Apple SUpport page, not from the AppStore.
- If your mac cam ewith Lion aor Mt. LIon preinstaled, there should also GarageBand been installed. Perhaps you are still having GarageBand 6.0.5 in a subfolder of Applications, or can download it from the "Purchased" tab of the AppStore.
- If your mac is brand new,and never had GarageBand '11 installed, you could buy the iLife '11 installer disk from Amazon. Install from the disk and upgrade from the AppleSupport page.
Have you bought the in-app purchase of GarageBand 10.0 or used "Restore Purchase)? (Expanding GarageBand for Mac (v10.0) content with an in-app purchase)
If you have done so, check, if you have a section with Legacy Instruments, and check, if this section is showing the "Deeper vocals" patch, like shiówn in my screenshot above. I did not have to buy the in-app purchase, and do not know, which instruments will be added by it.