Forza Motorsports fails to keep the controller active, even though its powered and speakers...

I only have one profile on the console.

The controller stays on when I play Forza 4, but not Forza Motorsports. I understand you're trying to offer general advice, but in my case, I shouldn't need extra hardware to make one game do what all the other games/apps already do.

The problem appears to be the OS relying on Forza Motorsports to disable the timeout, so the controller stays on (when it's powered by USB), but the game has overlooked this.

Forza Motorsports could be patched to behave correctly, but it makes more sense to fix this at the OS level. After all, if the controller is powered by USB, there's no reason for the OS to kill it so eagerly.

I could understand the OS turning (powered) controllers (or the entire console) off after three or four hours of complete inactivity (no input whatsoever), but it should never kill a powered controller while the user is actively using the console.

Alternatively, this could be added to the settings, so users could set the timeouts for controllers, or something similar. I don't have an exact solution in mind. I just wanted to raise the issue really.

The entire electronics industry has eliminated optical connections, it's not just Xbox

Very likely your issue comes from the fact that you have two different profiles signed into two different controllers, and since you're not using the controller your audio is plugged into it will continue to turn off

mini televisions these days have a headphone jack that you can plug your speakers into. If you're using a monitor without audio support, then you would need to purchase an external amplifier, or hdmi audio extractor

I just play a lot of Halo 5, and improve my skill in Halo 5, by playing Halo 5, but I do occasionaly play the Forza games, but when I use a controller, its more casual, I have a steering wheel for Motorsport titles that I am more competitive in, even though
the steering wheel is a disadvantage in the Motorsport games (or so I've heard)

Forza 6 achievement list:
www.twinfinite.net/.../forza-motorsport-6-gets-its-full-achievement-list

I would go for that game, but after failing Forza Motorsport 5, I probably would not understand the game.