CD-i member Muh Mullet posted some nice pictures of a true CD-i Karaoke Set: A CD-i 220 including the official Philips Karaoke Mixer 22ER9060, bundled in a metal case so you could transport it. Lots of CD+Graphics were bundled with it, most of them we've never seen before, but they are all perfectly compatible with your CD-i playwer, which is of course suitable for CD+G discs as well.
Muh Mullet: "here's something you won't see everyday: CD-i 220 rental Karaoke set,
consisting of: a Philips 220 CD-i player in a custom flightcase, with compartments for controller and RGB scart lead, power cord, and compostite video lead, RGB scart output, and Svideo output, it has a 50/60hz PAL/NTSC switch on the back and a battery holder installed. There is a set of CD-i karaoke titles and a binder with forms for contestants to fill out, as well as a list of all titles and songs to choose from, a Philips 22ER9060/00 Karaoke mixer with its original Philips microphones."
CD+G (also known as CD-G, CD+Graphics and TV-Graphics) is an extension of the compact disc standard that can present low-resolution graphics alongside the audio data on the disc when played on a compatible device. CD+G discs are often used for karaoke machines, which use this functionality to present on-screen lyrics for the song contained on the disc.
The CD+G specifications were published by Philips and Sony in an updated revision of the Red Book specifications. Their CD-i players were also all compatible to play CD+G discs, which made CD-i a suitable platform for karaoke as well. The first CD to be released with CD+G graphics was Eat or Be Eaten by Firesign Theatre in 1985. The CD+EG is a similar format that allows for better graphics, but has very rarely been implemented in releases.
[Thanks, Muh Mullet]