I don't know how Karaoke-files are structured but my guess is they might be similar to subtitle-files where a time period is specified and also the text (words) to show.
I don't think the mediaplayer has an event which shows the current position so I guess you can use a timer and call mediaplayer.position continuously and based on its position you read/analyse the karaoke-file and show the corresponding text related to the current position.