As you might know there was a sound replacement project that had replaced about half of the sounds, but then sadly enough died. Now OpenGFX seems to be nearing its completion, missing a free sound replacement set becomes a bigger issue.
As a result I've restarted the sound replacement project again. OpenTTD's (recent) nightlies (and the upcoming 0.8 releases) support loading custom sample packs in the same way as OpenGFX. This removes a major obstacle of the previous attempt(s): no real support of OpenTTD.OpenSFX development
Finally I have created, with the help of the #openttdcoop guys, a repository/tracker for the sound replacement pack. I've called the replacement pack OpenSFX and can be found at http://dev.openttdcoop.org/projects/opensfx
. I already added all samples from http://wiki.openttd.org/Sound_Effects_Replacement
that had a proper license and sounded decent. I also created a ticket/issue for each of the missing samples and hope that you can help finding/creating a suitable sample for the pack.
I suggest that we use the Creative Commons Sampling+ 1.0 License as all existing samples are released under that license, which makes it the easiest for all. Please mention the author and the samples you took and (re)mixed, so we can properly credit them too.
I am aware that a number of the existing samples aren't that good, so at the moment I'm looking for people who have some skill with editing/mixing/creating the audio samples that are willing to help to improve and finish the set.OpenSFX downloads
At the moment the latest 'release' is 0.2.0, which replaces almost all of the samples. You can download the release from either the in-game content download system or http://bundles.openttdcoop.org/opensfx/releases/
Nightlies can be found at: http://bundles.openttdcoop.org/opensfx/nightlies/