Would that not depend on the speaker itself? Surely the instructions will tell you how to do it. I would have thought that you just connect it like a BT headset and the phone will use the speaker instead of the phone speaker i.e. the phone won't know it's an actual speaker, it's just fling the data over the BT whch the speaker is playing..... After that, it's just a matter of using the phone to stream from your server in the same manner you are doing just now.
How much are Bluetooth speakers? How about a WiFi unit and cut out the middle man of the phone? I've got a squeezebox and its great.
WSB500 Portable Bluetooth Wireless Speaker (rechargeable)
Connecting to the speaker is the easy part
How do I stream from the network share on the phone?
Do you have an app that already streams it from your network to your phone just now?
Really liking the look of that, some cracking reviews too
There are different ways of streaming it, use ES file explorer and play via a network share or using DLNA with BubbleUPNP. Those are just a couple of ways.
Have you thought about moisture and the speaker?
Does the squeezebox run of batteries?
They can do. I have the Squeezebox Radio. Its significantly more expensive than just a BT speaker, but then it does more.