First up, you question; the answer your question lies in the README that comes with Synergy:
What happened to the installer?
Old versions of Synergy were distributed as a System Preferences pane, which meant that they had to be installed into a specific location in order for to work. Now that it is a standalone application it will work from any location and doesn't require an installer.
The underlying cause of this change is that Synergy 4.0 takes advantage of a new Leopard technology called Garbage Collection, and this is incompatible with the System Preferences application, thus requiring that Synergy adopt a new format and cease to be a System Preferences pane.
As for the volume key issue, I am not sure if I can reproduce the same issue, but if I press and hold for a long time I do observe some "jitter" in the volume. I'll investigate and see if I can get out a fix within the next few days. Thanks for reporting the issue.
Ok, I've just committed a workaround/fix as described in my previous comment.
Basically, whenever you press either of the volume keys it resets the internal state (ie. if it was waiting for a "release" event because it saw a "press" event but the system never sent the "release" event, then it stops waiting for it).
So even though the keys may jam in effect you're unlikely to notice because the volume will hit either the superior or inferior limit and stop moving anyway, and the next time you press a volume adjustment combo things will get reset.
So I don't know why the events are getting lost, but at least the program handles the lost events a little bit more nicely now.
Expect to see the fix in a new release within the next few days.
I've just backported this fix to the 3.4 branch, and will be doing a 3.4.1 release very soon, so that people who don't want to upgrade to the 4.0 series yet can use 3.4 on Snow Leopard without any problems.