T&C state only for new customers, but we were able to get it to apply to existing accounts, so YMMV
Whether your holiday travel plans involved being stuck in traffic, waiting at the airport, or jogging at the gym, you can transform them into enjoyable “me” time by stocking your digital bookshelf with Audible, the popular audio entertainment platform easily accessible from your smartphone, tablet, or MP3 player. Enjoy one audiobook of your choice each month for free with this offer:
To enjoy, claim your offer and redeem online by April 30th, 2015, at 9 p.m. PST
Two Months of Audiobooks Free! No purchase required. A typical trial is only one month, so this offer is especially deal-licious!