There is a post in the Forerunner 945 Forums requesting playback speed for podcasts and books, thought I'd add the request here as well for 645M. Almost all the people I know listen to some or all of their podcasts at higher speeds. Similar for books.
Right now there is a premium app for podcasts that will do this for you called runcasts. Its a pretty slick app - it compresses the files and changes their speed then syncs to your device. My main problem with this is that it requires manual management of episodes and podcast subscriptions. This is compounded by the fact that runcast doesn't sync to any other device, so there is no way to automatically remove it if you've already listened to the podcast on your phone, you have to manually do that yourself. It's also $5 a month for this feature - I think its a nice app with a clever solution to this playback problem but paying $5 a month just to get playback speed with the added overhead of manual podcast management is steep.
The solution for books is similar, you use a converter to re-record the book at the desired speed and then upload that book to your books.