My question is prompted by watching some old movies on TCM: movies made before 1950 frequently have very noisey soundtracks and voices are difficult to understand. I understand that the old “seleno phone” modulated light soundtracks were superceded by magnetic sountracks in the 1950’s. What do they do when making a duplicate print? Can old optical soundtracks be cleaned up? I understand that removing a constant frequency noise 9like 60 hz hum) is pretty easy-but what about the pops and static on these old sound tracks. i was watching the old Bogart flick (“THE MALTESE FALCON”)-some of the dialoguwe is just about gone. 
Most Hollywood soundtracks from the late 1930s onward are preserved as three separate elements: dialogue, music, and sound effects.
Today it’s fairly easy (if time consuming) to eliminate “pops” and other artifacts from movie soundtracks, using a graphical representation of the soundtrack as a sound wave, where the artifacts are visible anomalies, and can be digitally erased.
I’ve seen The Maltese Falcon several times over the years, and the soundtrack has always sounded healthy to me. If the image looks fine, so should the soundtrack sound fine: the nitrate masters were made at the same time.