It is possible to forward all audio to the Android smartphone via wi-fi? Are there apps/methods that allow for the sound to be forwarded to the smartphone?

So if I would watch a YouTube video on the PC, I could also hear the sound via the smartphone.
One way to do this is to use Airfoil Speakers for Android, which is free, but requires a server component (Airfoil) to be installed on your PC. The free version plays audio fine for 10 minutes and then overlays noise. The license key is $25.