I am currently working on improving the sounds in Pak128.Britain-Ex; the sounds in Simutrans generally are somewhat neglected, being some very old and rather low quality sound files that are often too long and somewhat irritating. I have been trying to replace the basic UI sounds with better sounds, replace the background sounds with less irritating ones (I did that some months ago, in fact) and slowly add realistic vehicle sounds for all the vehicles (which this pakset had hitherto lacked, except for aircraft).
I should be grateful for any feedback on what I have managed to add so far.
Also, if anyone would like to help to improve the pakset without knowing how to code or produce graphics, finding sounds, obtaining permission to use them (if they are not already released under a suitable licence), and editing them to the appropriate length (for vehicle sounds, 3-9 seconds, normalised, with a short fade in and a long fade out; editing can be done in Audacity, which is free and works with both Linux and Windows) would be very helpful. There are lots of vehicles which have more or less unique sounds (especially internal combustion engine land vehicles) which many familiar with those particular vehicles will recognise and it would be helpful to have unique sounds for each. Other types of vehicle, such as steam locomotives and, to a lesser extent, aircraft and electrically powered land vehicles, can share a smaller number of sounds much more easily, although we are still missing some generic sounds such as for old trams and trolleybuses and various electric trains.
A rich source of suitable sounds may well be Youtube videos, whose creators may be more than happy to let extracts of their work be used for free in a non-commercial open source game: I have already had permission from one Youtube user who makes high quality videos of 'buses, to distribute sound clips from his uploads under the Artistic Licence 1.0.