Mr. Smith Goes to Washington (1939)

The 1995 LaserDisc sounds great -- much less noise reduction than the DVD and blu-ray. The UHD disc uses the same audio track as the blu-ray.

(I think the DVD track I was sent is an AC3 to AC3 transcode, but whatever. It's a bit less muffled than the blu-ray, but nowhere near as clear as the LD.)

Update (01/01/2021): Added the first disc of the 1985 RCA/Columbia LaserDisc, which should be enough to make judgements on its overall character. It's not really any better than the 1995 LaserDisc -- I hear a lot more distortion and certainly a fair bit of compression (I don't know much about the analogue LaserDisc transfer process, but if it's anything like cutting a vinyl record, adding some analogue compression would make the process easier, especially if there's too much information to fit on a single disc). The opening music sounds much worse on the older LD, but after adjusting for volume the meat of the film sounds similar but with more distortion.

Mr. Smith Goes to Washington (1939)

The old Columbia DVD* and the Sony blu-ray sound very similar--not particularly great. It's clear that both come from the same analogue source and that both have been subjected to a strong amount of filtering. All frequencies above 7.5 kHz on the blu-ray have been chopped off.

*The one not part of the Capra box set, although there's no reason to suspect the box set DVD is any different

I suspect the old Columbia Tri-Star LaserDisc has the least neutered audio track, assuming it uses the same Library of Congress restoration as the DVD.