My thoughts, things where there was a difference I thought worth noting:

• Train in tunnel osfx05 misses the nice reverb from the original TTD sound (OpenSFX also doesn't have great reverb, although it has some). Perhaps this would be a good opportunity to improve it? Also both of the train sounds are quieter, and IMO osfx_04 would benefit from a little bit of motion sound on the end as per the original.
• osfx_33 sounds better to me than the original.
• osfx_37 sounds a bit artificial, like it was a signal generator rather than a real cow. (The OSFX sample could do with replacing though, it has extraneous noise and and obvious click)
• osfx42 now has a bit too much of a "sci-fi" tone? I think the bottle pop is good, the lead-in tone needs something that's less of an obviously synthesised sound.
• A technical issue - osfx_54 is clipped (the waveform is truncated). osfx_55 sounds nice!
• osfx_59 (supersonic) is a huge improvement! The old one was clipped and had noticeable noise reduction artifacts, this is much better.
• osfx_61 doesn't feel at the right phase of the flight, is there a more "takeoff" type sample available? The original TTD sample is quite different to OpenSFX, more of a turbofan whine, so there's some scope here to do something that doesn't have to sound the same IMO.
• osfx_66 is a little longer than the original, not too much. There is an opportunity here, though - TTD had some nice "tropical" sounding bird samples, I feel the ones in OpenSFX are a little too "European". Not an essential change :)
• osfx_69 sounds like it's from an RC plane, and has some bad reverb effects at the end and some random electrical noise at the start. This one I think needs a much better sample.
• osfx_71 is an improvement IMO, I never liked OpenSFX's monorail though. The tail of this sound does feel a bit over-processed to me, it has a repeated modulation which is quite noticeable.
• osfx_72 again a good replacement, but worth checking if there are some more tropical birds which could be used.

Note I have force-pushed a couple of times and updated various sounds, so please check again.

• osfx_05 now has a simple echo. Maybe not perfect yet, but still
• I agree that osfx_61 still needs work, I don't like it either. I just totally forgot about it …
• The cow sound (osfx_37) ... It didn't occur to me that this sound sounds "unnatural". It was a real cow.
• I will look into osfx_54. But I'm afraid I have to throw away this sound :(
• For the tropical birds (69 and 72) I used the "similar sounds" feature of Freesound.org. I believe it's even the same bird species than before! :D I don't want the birds to be too "prominent" either, just a simple background noise. Personally, I like those birds.
• osfx_42 and osfx_43 are the toy factory sounds, and yes, they did not turn out well at all. It was my failed attempt at redoing the current OpenSFX sounds. The challenging part will be to make the sounds sync perfectly with the toy factory animation
• osfx_69 is the toyland propeller plane sound, so it is intentionally "wacky". But maybe there's a better sound for that? Hmmm …

I also strong recommend to actually test the sounds in-game. OpenTTD seems to alter the volume of some of the sounds, and this can have a big impact.

Updated: osfx_61, osfx_54

Ah yes, that makes a lot more sense as a Toyland sound. I wonder if there's a way to get a "clean" version without the extraneous sounds. (Are they different enough in tone that a spectral repair would work?)

The echo improves 5 a lot.

The difficulty sometimes with sounds is even with a real source, noise reduction and repair can remove enough to make things sound like a pure tone. OpenTTD can tolerate noisier samples because it's not doing too much with them; they're mostly just ambient sounds and not processed further into a mix where very clean originals are desirable.

Updated toy factory sounds osfx_42 and osfx_43. This sound pack is now ready for re-review.

I just replaced the railroad crossing sound osfx_14. It turned out the source sound was based on a non-free sound (-NC), so it had to be replaced. The previous sound was a bit low-quality anyway (noise).

Okay, I added myself to the credits and change the commit names. Anything else?