Laserdisc problem encountered----Help?

I went to play a laserdisc last night, and discovered no sound, but the usual great picture. I checked all the wiring connections, and tried again. Same thing, so I tried another disc and the same thing occurred. I then started going thru my collection, and out of 8 discs I tried, 3 had no sound. The discs are in flawless condition, not even any fingerprints on them. Today I'm going thru my entire collection, but my question is, has anyone else encountered this phenomenon?
I thought maybe the laser might be going bad, but why does it play fine on some discs, and not on others?
Would appreciate hearing from anybody else who might have run into this.