How to play YouTube or any audio from my desktop Linux and send the audio playback to Android?
Couple of different ways:


Since you're specifically talking about Linux, you can download Remote Audio from Google Play. It's free, and connects to your Linux computer using SSH, then listens to your soundcard via its dev/dsp mount.
Download a "Remote Desktop" app. There are several, most are paid apps, and they each target a particular remote admin protocol and/or desktop OS. Most of them will forward audio as well as video, turning your Android device into a full remote control for your desktop.