I have some time this week, so I want to help contribute if I can. From what I gather:
 1. Even though it isn't as good as producing our own sounds, we can use sites like freesound to get sounds, as long as it's CC-BY-SA or sampling+.
 2. The sounds that need to be replaced are blank/[] under the "replacement" column at 
http://tremulous.net/wiki/Sound_Licenses .
 3. We have to submit them for review over at 
http://tremulous.net/forum/index.php?topic=14952.0 .
So I have a few questions. 
 1. Where are the sound files located? I tried poking around the pk3 files in the gpp directory but didn't find anything. 
 2. Other than replying with links to the files in 
http://tremulous.net/forum/index.php?topic=14952.0 , is there anyone in particular that does a QC on them?
 3. How can we test it locally? Do we replace the sound files with our own, create our own server, and then test it?
Edit:
  There is not QC at the moment.
  Sound files are located in base/data-1.1.0.pk3/sound , .pk3 files are .zip files.
  You have to create your own server to test sounds, joining US1 server gives an error.
  You can test out weapon/alien sounds by doing something like /devmap atcs to allow cheats, then /set g_alienstage 2 to get stage3, then /give all to get credits.