Some LDs contain Dolby Digital or DTS sound. DTS sound is available on the standard optical digitial audio out but DD sound comes from an RF modulated connection that you can't just stuff into a digital input. You will need an RF demodulator which then provides standard digital audio.
An additional bit of trivia is that there are 4 audio tracks on a typical laserdisc. Two of these are analog tracks and two are digital. The analog tracks are provided as a fallback in case a player doesn't support digital audio tracks.

On a DTS laserdisc, the digital tracks are used for DTS encoded audio. So if you have a player which supports digital audio but no DTS decoder, you are stuck listening to analog tracks.

On a AC-3 laserdisc, the digital stream is RF modulated into the right analog track. So if you have a digital player but no AC-3 decoder, you can at least get Dolby Surround from the digital tracks, but no Dolby Digital. If you have a player which is analog only, you get only a mono analog track. Worse, if you have a player which not only doesn't read digital tracks but isn't even AC-3 aware, you get the one analog track on the left side, and a hideous screeching on the right side.

I don't remember if any actually existed, but it would be possible to have a LD with both AC-3 and DTS audio. The problem is that if you don't have a DD or DTS decoder, instead of falling back to Dolby Surround, or even to stereo analog, at best you get one analog channel, and at worst you might have to deal with screeching.

Any wonder why LD was rejected by consumers? Besides the inconvenient size of the media, short lifespan due to oxidation of the reflective layer of many mass-produced discs, composite video, and complex equipment which had high production costs. Oh, and having to flip/change discs multiple times in the middle of a movie, though some high-end players would omit the flipping stage by reading both sides. Some LD players doubled as 5-disc CD changers though, like the one I have (Pioneer CLD-Mxxx series); that's rather convenient in terms of rack space, but still nothing like the storage density of a DVD. Too bad my DVD collection has begun to rot with oxidation too :-(