If not mistaken this function never work. Why not you call Sony to verify

Where is the mic port?

Don't think android has this function as well

I am using mi box 

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have u try mic with audio from TV via digital out?

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that price is for minibox or xiaomi box?

lazada shot up to 249 today

Have you ever disconnect the Line Input from the PC and use only the mic just to confirm this?

Yes, audio. The Line Input is normally refers to the RCA type connector. The signal level (Line Level) is normally spec at 2V peak-peak for consumer product. This input is colored WHITE and RED for Left and Right audio signal respectively. The YELLOW color is normally for composite video. You can see this at the back of your TV.

Since you can use the mic when you removed the RCA connection to your HT, why not just buy an RCA connector switch. You can buy the one with remote control if lazy to move. So you can connect and disconnect the audio cable from the PC when you want to karaoke or want to use source from PC? 

I meant disconnect the audio only and keep the video. Try if this can work. I believe you have Red, White and Yellow going into your HT

Like I mentioned before, the White and Red wire is for audio Left and Right respectively. The Yellow one is for Video. Actually, I'm a bit blurred on the connection between your PC to HT.

You play your karaoke video using VLC on PC. You also mentioned about USB cable. What I presumed was the audio and video from PC was using USB to HT unit. A bit strange for me for video using USB even though I believe that there is such cable.

How do you connect the video to your TV? Direct from PC or though HT (if your HT has video in)? Using HDMI?
How your audio from PC is connected to your HT?

My suggestion was that to leave the video cable untouched but get you audio L & R (White and Red) goes through switch box before connect it to HT. By doing this, you can cut off the audio in when you want to use the karaoke function. You can switch it back on to connect the audio if you want to have the audio coming from the PC.

This was my idea since you mentioned that if you disconnect the audio (White and Red cable), you can get the karaoke function.

what if audio cable plug to TV instead?

Just as I thought. The USB from PC is providing just stereo audio (L & R). So, as you mentioned before (if I understood it right), the mic will function if the USB (on PC end) or the Red/White RCA connector (on HT end) is unplugged. So, the karaoke video will still work through PC to TV via VGA cable.

So, instead of plug/unplug the Red/While cable at the back of the HT unit, consider buying a switch that can connect/disconnect the audio cable from PC to HT. I know that there is switch that can be remotely controlled out there. Maybe it has ability to switch more than just one stereo port. If too much for investment, you can just buy a manually switched one. Probably around rm10 

Basically, the switch will disconnect the audio (the USB->Red/White) and you will have the karaoke function with mic on HT with karaoke video played from PC. If you want to watch movie from PC, switch to reconnect the audio back to HT and you have normal HT function. 

Isn't it mentioned in the manual (provided by shinchan^^) that the karaoke function is for playing from usb device or disc? (and never mention other devices such as PC)

Not sure what files you use, but wont it be easier for you to copy the karaoke files to a USB drive/HDD and play it from the HT system? (Or use the DLNA option as suggested by Zot)