streaming – is it possible to connect another android device for “media audio” thru bluetooth?

I can connect to a notebook, and on android it says “connected for media audio”, perfect! But i cant walk for hours with a notebook…

I got another android on a smartphone (B) that cant run games and have a small screen to watch movies properly. But the bluetooth connection is only for internet sharing. Why there is that restriction? It would be so much easier if they just let it work…

So, if there was some way to tell my good smartphone that my bad (B) one is a desktop, (B) could be used as “media audio”.

I need to send all audio, from any app playing audio. For ex, some times i play a game with its music off and use mxplayer playing musics that i like at the same time, and also need any system sound to go thru.

I guess i will need to root the (B) phone to let it pretend to be a desktop? Most probably it is just a simple thing like “user agent” on browsers right?

Related:
How can I stream audio from another device via Bluetooth? (the answer has a broken link. Why was it upvoted if it isnt useful?)

I think thru wifi would be more complicated? But if you have a how to link i may try it too. The problem is that i need the wifi for internet and i may not be at home. But i think this should be another question if thru bluetooth is no go.