This would allow multiple devices to sync to the main database online and users could choose which devices to use to listen to. If I'm on my computer I just load up the website interface. If I'm in a reading mood I can be on my tablet, then when I feel like walking I can pick up my phone and continue listening where I left off on my tablet.

The business side exists too. As users sign up your organization will obtain tons of customer and user data to better target ads, promotions, and podcast suggestions. Email newsletters that talk about new podcasts, or new episodes to user's subscriptions would help your engagement numbers and provide an avenue for upsells.

If the synchronization is a difficult piece to build you could charge a small monthly subscription $5/month...

Just a thought.