There are a few possible reasons why your voice might not sound as good when singing karaoke as it does normally. One possibility is that the karaoke machine is not tuned to your voice correctly, which can make your voice sound higher or lower than it actually is. Another possibility is that you are not used to singing with the karaoke machine's backing track, which can throw off your timing and rhythm. Finally, it is also possible that you are simply more nervous when singing karaoke, which can affect the quality of your voice.