First a short explanation of the Digital Video cartridge: The dvc is a cartridge that gives the CD-i player MPEG full motion video compatibility as well as extra memory. It goes in the extension slot. Most titles indicate whether they require the cartridge or not. The easiest indication is what it says on the bottom of the front boxart: 'compact disc interactive' means it is a base case title which does not require the dvc. 'digital video on CD-i' titles do require the cartridge.
CDifan: "I would be pretty surprised at CD-i titles combining music with sound effects without a DVC not being mono. It's technically possible but would take a lot of CPU resources. "Digital Video on CD-i" was mostly used for "movie" discs using the CD-i specific format instead of Video-CD. Technically, the DVC adds an MPEG sound path to the regular CDDA/ADPCM sound path, which means that sound effects become easier. If you have only one path, the sound effects need to be mixed in software into the music sound data before it is played by the hardware, which costs a lot of CPU. Technically the sound effect data could be mixed both into the ADPCM and into the MPEG paths (CDDA data never reaches the cpu), but I have never heard of a title that mixes MPEG (technically hard due to compression of the composite sound data). Mixing into ADPCM is easier because the compression is per-channel; if you want to pay the bandwith/CPU/quality price ADPCM can also be reduced to PCM by setting all filter values to zero. Note: Both sound paths (MPEG and CDDA/ADPCM) have two channels but these cannot be separately played."
[Thanks, Naean, retrostuff, cdifan]