This app was mentioned in 6 comments, with an average of 1.00 upvote
I don't think this is possible with Bluetooth, unless you have speakers that specifically allow this, like these. Neither it is possible with DNLA. For both, the protocol does not allow it.
If your speakers support AirPlay and you have a WIFI-Network present, it is possible with a rooted phone and AllStream installed.
If you are rooted. I think you can cast Audible with AllStream. I did it once before I got a new phone and lost root.
https://play.google.com/store/apps/details?id=com.kineticgamestudios.airtunes.android&hl=en
I have no experience with it, but this seems to do what you are looking for.
I got this app too but related to 'AllStream' for Airplay/DLNA on Android.
https://play.google.com/store/apps/details?id=com.kineticgamestudios.airtunes.android
Update - and sadly I might have spoken out a bit to early.
On my phone the first app I've linked to (Airaudio) produces micro stutters every few minutes, so I looked arround for alternatives. There are two.
Allstream https://play.google.com/store/apps/details?id=com.kineticgamestudios.airtunes.android
and
Bubble UPNP http://repo.xposed.info/module/com.bubblesoft.android.bubbleupnp
Both produce the micro stutters as well (If you listen in on the music you will catch one every two songs - but its barely noticeable, and from what I can tell, might very well be dependent on how your phone behaves while hibernating), but less pronounced. Also, I actually prefer Allstreams minimal interface over Airaudio any day.
(To access settings in Allstream, hit the menu button on your phone while it is launched - and I hope a virtual button will be part of its minimalistic interface, If your phone doesnt have a menu button anymore.)
So before you spend the money - maybe try the three solutions independently. They all are working in a "demo mode" before you purchase the license.