Various rules apply, including that your must be a US customer at Amazon and the voucher expires if not used by Dec 2.
If you wish to give an album as a gift, the credit will be applied; you can see it on the very last page before you finalize the order (unlike with book gifts, any gift balance is used first). I also suggest you first check your credit balance in the MP3 store (you can do this on any song or album, there is a link just under the buy button) and use up any existing credits first, so that they don't try to mix with the voucher. I've had trouble with this in the past and had to get Amazon involved in order to get my credits back.