Valid for audible.com accounts with non-active memberships.
Edit: Updated link for previous Audible members. The link will prompt you to login. You can also enter the code on the desktop site, by adding something to your cart then clicking on "Do you have a promotional code?".
https://www.audible.com/ep/speedpass?ipRedirectOverride=true works if new to audible
Note: Updated terms state "Valid for US customers only." but the audible part of the offer at least, is confirmed to work on Australian accounts.
Terms: This is a limited time offer. Offer valid from April 1, 2018 12:00AM PT until July 10, 2018 at 11:59PM PT or while supplies last. You will receive a FREE two-month Audible membership that includes 2 credits as part of the standard Audible Monthly Gold Membership plan. After the first 60 days, this membership will renew monthly at the regular price of $14.95. You can cancel at any time and you will not be charged thereafter. After you successfully sign up for the FREE Audible Trial, and your account is verified, within 2 weeks you will receive a follow-on email containing a unique code for a $10 electronic Amazon.com Gift Card. The Amazon.com Gift Card is non-transferable and may not be re-sold. This offer is limited to one per person. To qualify to receive the Amazon.com Gift Card you must have an existing Amazon.com account that was opened prior to March 31, 2018 and that account must have been active (as evidenced by at least $1 in transactions) during the six months up to March 31, 2018. The Amazon.com Gift Card only applies to products sold by Amazon.com or Amazon Digital Services LLC (look for "sold by Amazon.com" or "sold by Amazon Digital Devices LLC" on the product detail page.) Products sold by third-party sellers or other Amazon entities will not qualify for purchase by the Amazon Gift Card, even if "fulfilled by Amazon.com" or "Prime Eligible". Offer does not apply to digital content. Offer may not be combined with other offers. Shipping charges and taxes may apply to the full value of discounted and promotional items. Offer discount will be allocated to the eligible item in your order. If any of the products related to this offer are returned, your refund will equal the amount you paid for the product, subject to applicable refund policies. Unless an Amazon.com Gift Card is the stated benefit of the promotion, promotional codes or credits (including those placed directly in accounts) may not be redeemed for Amazon.com Gift Cards. Offer is limited to one per customer and account and may not be combined with other offers. This offer is non-transferable and may not be resold. Audible reserves the right to modify or cancel the offer at any time. If you violate any of these terms, this offer will be invalid. By signing up you agree to Audible's Conditions of Use and Amazon's Privacy Notice and authorize Audible to charge your designated card or any other card on file.