What is the main modem router? Make and model please.
Is this your main wireless access point?
What is the Express configuration now in the network?
Is the Apple TV plugged into the main router or the express?
Are your non-wifi speakers actually airplay capable? Or do you need an airport in front of it to pickup airplay.. this is crucial point. Please test the speakers now.. plug them into the main router.. run iTunes and see if the speakers are recognised as airplay device.. if not you MUST use an Express.
The Apple TV cannot act as a base station. But if you don't need the express for the connection then you can reuse it in the bedroom.
There are a couple of alternatives.. but none are without paying out some money. In fact more than buying another airport more than likely.
1. Get ethernet cable properly installed in the wall.. hugely superior to any other solution.. and the most expensive.
2. Powerline adapters .. these are called homeplug in US. A set of cheap ones might be $40-60. Connect one end at your main router.. and other end in your bedroom.. but it might still need an airport to work. You also might do better buying a third unit and plug that next to the Apple TV so everything is ethernet.. the Express can be used as needed then for wifi or airplay.
3. Buy a second hand airport extreme gen5. Plenty around and cheap.. hugely superior wireless to a brand new express.. albeit they will be 5-7 years old already and lifespan is over.. in terms of if it gives trouble.
Plug that into the apple TV as long as it connects to the main router by ethernet NOT wireless which only the express can manage.
🙂 Tesserax.. you son of a gun.. 😉