My personal thoughts on these:
A) This is generally something you should explain regardless of whether you're planning on using it as an incentive or not.
B) This will mostly apply only to musicians who are interested in singing. Depending on your intended audience, this may or may not be desirable, and depending on your target amount, this may or may not be plausible.
C) This is a general incentive but may also easily scare people off under the belief that you are a scammer. I would start the study and then as participants start dying down, introduce that incentive (and provide it to people who have already completed the study, for fairness).
D) This is great for ease-of-access, but not necessarily an incentive. I would highly recommend allowing access on a smart-phone at least, since phones are guaranteed to have microphones of decent quality.
E) This is also for convenience.
F) This is dependent on the purpose of the research and on the individual person.
G) Consider your audience. If your study is designed specifically towards musicians, offer a chance to have a free vocal lesson, rather than simply feedback, or perhaps access to several tips to improve singing. If your audience is a general audience, consider accessibility, convenience, and rewards similar to the $500 gift card, but be wary that you don't want to come off with a, "Too good to be true," vibe.
