I stumbled across this and tried it out on my Nvidia Shield and it worked great! It has a really deep library of all the "Sunfly" videos that you see at various Karaoke bars.

Steps are simply:

1. Install "Kodi" from the Google Play store- Kodi is a fork of XBMC and is a general media console app that I recommend for lots of video streaming and playing local video on your android tv.
2. Kodi lets you install addons from various repositories. Add the "Xunity" repository (details in the link provided)
3. Once you've installed the Xunity repository, install the addon called "Mikeys Karaoke". It has a library of sunfly videos, as well as some sort of index of YouTube karaoke videos. It's all streamed so you don't need to have a big hard drive or anything. I'm doing it on the Nvidia shield but you could do this easily on the Nexus Player.

I'm even thinking of getting some sort of mixer so I can use a microphone with it.

It was super easy to setup- if anyone else is interested or has any questions, feel free to reach out. Enjoy!

Musicxmatch also has an extensive library and provides lyrics.

It's a neat add-on, but for those with a more unorthodox choice of karaoke songs may find the song selection in this add-on a little lacking.

Kodi had karaoke support baked right in (provided you had the CD+G zip files) but was removed in v16 Jarvis. Those running Jarvis or newer, this plugin is your only option. Native karaoke support was removed because it is no longer actively developed and no one knew how it worked. If you're running pre-jarvis, you can enable karaoke support via the audio advanced settings in kodi

Another gotcha is I haven't found out a way to enable USB microphone pass through on Android TV.

Because of both of these reasons, I keep a pre-jarvis build of kodi running on my i3 HTPC. It's sole purpose is for karaoke parties my wife and I host once in a while

Would like to have something like Singstar on Android TV.

If you want to use a wireless microphone with your speaker set-up you need to purchase a mixer, a set of wireless microphones with their hub, and an RCA to 6.35mm cable.

1. First of all if you want this to work you need to find a way to simultaneously route the audio from the microphone and from your device to the mixer; I've no idea how to do this solely with a receiver so I make use of pass-through technology.

2. I have my Nvidia Shield connected to one of my HDMI ports, and the receiver connected to the "ARC" port. This allows DTS pass-through for the times you want to enjoy high-quality sound/surround sound from your Plex/Kodi/SPMC library.

3. Now that you have connected your device to your television, and you have the receiver in the ARC-Capable HDMI port you're going to need to route the sound from the Television's connected device (Shield, Apple TV, Chromecast, etc) to the mixer via the audio out ports on your television (Usually RCA) with the RCA to 6.35mm cable, and the sound from the microphone's also to another port on the mixer.

4. Read the manual, mix the two sound sources, get the echo and special effects to your liking, and make sure the "sound mode" on your receiver it set to coaxial/analog then you should hear the sound from the connected device along with the microphone. Ensure all the connections from the TV/Device/Mixer/Receiver are routed correctly.

5. Fire up Youtube or your favorite Karaoke service and get to singing!

Note* Enabling ARC (if supported) allows the Television to route the digital signal from your device (pass-through) directly to your sound device (receiver, speakers, soundbar) which allows for higher quality and DTS if you have that set-up.

Disabling ARC is required if you want to use the microphone's output in conjunction with the TV/STB's sound output. The mixing of the analog signals which is then routed to the receiver is what enables a Karaoke mode. Enabling ARC makes it so that the digital signal from your STB is routed directly to your receiver with no middle man which allows for DTS and higher sound quality.