what about speakers? could i direct sound to individual speakers or will it just play all at once from a central source?
One central source.
maybe not 50. maybe not 30. i've got a length of 25 feet by 5 high, so a dozen and a half might be enough.
would a projector work with that kind of aspect ratio (1:3.75)?
That is a pretty large size and quite an aspect ratio. You may end up needing 3 16:9 projectors. If that is the case, the sound of each section would be sent to the related third of the display if each one was run by a cheap computer. You could even make one of the videos in each section large with smaller videos around it. The sound could be louder on the larger video and they could rotate in. It would give the effect of everyone talking together but as though you were in "conversation" with only one. Having three sections allows for three people to have such an experience at the same time.
Using that same layout idea, you could use a larger monitor for each section with smaller 7" or so monitors around it. Some could be connected to a computer while others to CD output. The same CD output may be able to be split and run into each section, removing the need for some of what may take many devices. This would remove the projection space needs making for a flatter display. Not sure how this would effect cost. Making the videos cycle in and out may be tricky, but may be just a matter of making synced videos that change offset to make it appear they are cycling via a central source.
Not saying this is the right direction for the project, just more thinking out loud. You most likely will end up with something very different than this in the end :).