The print I had was 24fps. For 18fps, what they did was remove frames, so when played at 18fps, it would run the normal 20 minutes. Since 24/18 = 4/3, they would have removed 1 out of every 4 frames. In Super 8, 400 feet is 20 minutes at 24fps, so at 18fps, it would be 300 feet. Don't remember the exact prices, but it should have cost 25% less. I do know they were charging around $40 in the mid to late 70s for L&H 2-reelers (19 - 22 minutes.)
Most people shooting their own sound films back then in Super 8 usually used 18fps -- that's why just about every projector had at least 2 speed settings: 18 fps and 24fps.
In short, the voices obviously were not slowed down, and recording the sound at 18fps apparently didn't affect the quality. Except for airline prints, which used optical sound the same as for 16mm and 35mm, Super 8 and Standard 8mm used a magnetic stripe (tape). I don't know how that compares to what speed tape cassettes were played at. I don't know how the quality of optical sound would be at 18fps. I do have a solo Super 8 magnetic sound Stan Laurel short with a music track, with a note that says to play it at 18fps, and the music sounds fine.
I once read, from someone who had DIRTY WORK at 18fps, say that both the picture and sound were still smooth. I believe Blackhawk did this for BUSY BODIES, too. They said they picked the most popular titles; that's why in my mind DIRTY WORK was always popular. I still have all the old catalogs I received -- but it's actually easier to look online now, since the Hugh Hefner website (believe it or not) has many of them scanned, going way back to the early 60s, and maybe some from the 50s as well.
Edit:
See page 87:
https://web.opendrive.com/api/v1/download/file.json/Ml8xNTEyMTc5NDNf?inline=1I can only find DIRTY WORK listed for 18fps. I never remembered BUSY BODIES being released that way but someone claimed they had one. And they only discounted the price by 15%: $34 vs. $40.
The first sound movies had sound recorded on disc, with the turntable synced to the projector. That's when they decided on 24fps -- so the frame rate had nothing to do with the quality of the sound. That's why they were later able to convert the sound from the discs into optical sound, and still project at 24fps. Several of the first L&H shorts were recorded on disc, and later printed with optical tracks. The downside is they had to remove some of the picture on the film to leave room for the optical track. The latest L&H restorations, I believe, use the full frame aperture with the recorded sound for those early shorts-- something not done back in the day when printed with optical sound.