Hello!
I'm presuming the fireplace will not be used?
You can get quite subtle trunking that sits on the surface of the wall ("d-line", I believe). I think you can paint over it too so it blends in better with the wall. Several people on here have used it with good results. If you do a search for "trunking" you will probably find a few examples
My guess is that it would look a bit odd if you have your TV covering some of the fireplace, but that's all it is - a guess. Any way you could set it up temporarily to see before you commit?
Regards to fitting all your kit in the fireplace... and assuming you don't want to hack out more of the back of the opening... you could build out the sides with some timber/MDF/plasterboard and put some shelves in, depending on your budget and DIY skills. Long term, some kind of floating wall might be effective if it's your cup of tea, there are some excellent examples on here, some real skills on show!!
I have never tried it, but personally I would be reluctant to have a speaker in there, wouldn't it get a bit "boomy"?
Probably not much help, but hopefully I have provoked the thought process a bit
Cheers!