Amazon's audiobook platform Audible is expanding its subscription lineup with the launch of a new Standard membership plan, the company announced Tuesday. It's a lower-priced option designed for listeners who want access to audiobooks and podcasts without paying for the company's premium tier. In the US, the Standard plan costs $9 per month and includes one audiobook selection, which remains available as long as the membership is active. The plan also includes unlimited listening from a curated library, which Audible says includes a selection of Audible Originals and nearly 200 popular titles previously available on Wondery Plus. By comparison, Audible's Premium Plus plan starts at $15 per month and includes a monthly credit that can be used to purchase any audiobook in Audible's catalog. Those purchased titles remain in a user's library even if they cancel their membership. The launch comes as audiobook services face growing competition from both streaming subscriptions and library-based apps.…