With TubeMate application, Android user can play Youtube audio in background.
1. Download Tubemate app from http://tubemate.net. I choose Android Freeware.
2. Install the downloaded apk. Open the app, press Next in wizard until end.
3. Press Close or go to Preferences to set up settings like download folder.
4. Sign in to Youtube.
5. Click on green arrow on top of bar.
6. First select the video quality then choose Play (Audio).
7. It will play audio only and user can play it in background or lock the screen.