hmm, if I understand your suggestion correctly, either red triggers a sound that plays for everybody or blu triggers a sound that plays for everybody.
I'm wanting something different. I'd like anybody to trigger a unique sound that plays only for red, and another unique sound that plays only for blu. Is that what your suggestion does?
Um, sorry for bumping this, but I think I can help, assuming you haven't solved this already.
I'm not that good with Hammer, but anyway, I think you're going at this from the wrong direction.
I succeeded in making sounds play for one team only, by working at it from a client side perspective. There is a console command, the 'play' command which can be used to play any sound in tf2 for that client only.
So, as an example, follow these instructions:
First, create a simple room (light, etc, the team-spawn entities).
Then, create a trigger multiple in the centre of the room. Name this trigger1.
Then, create two new trigger multiples that encompass the whole room, and name them both trigger2. Set them both to start disabled. Then, give one of them a filter name of bluefilter, and the other of redfilter.
Then, create two filter_activator_tfteam entities. Name one bluefilter, and one redfilter. Set the team according to the name.

Make sure that the filter mode is set to ALLOW entites that match that criteria.
Next, create two point_clientcommand entities. Name one sound1 and one sound2.
This should be all the entities you need if I've got this right.
Now it's onto the inputs/outputs stuff. Select trigger1, and give it the following output:
Code:
My output named: OnTrigger
Target entities named: trigger2
Via this input: Enable
With a parameter override of: <none> (although this box is actually automatically greyed out for this one)
Then comes dealing with both the trigger 2's. For the one specifying the filter of bluefilter, give it the following outputs:
Code:
My output named: OnTrigger
Target entities named: sound1
Via this input: Command
With a parameter override of: play vo/announcer_success.wav
Code:
My output named: OnTrigger
Target entities named: trigger2
Via this input: Disable
With a parameter override of: <none> (although this box is actually automatically greyed out for this one)
Now deal with the one specifying the filter of redfilter. Give it the following outputs:
Code:
My output named: OnTrigger
Target entities named: sound2
Via this input: Command
With a parameter override of: play vo/announcer_failure.wav
Code:
My output named: OnTrigger
Target entities named: trigger2
Via this input: Disable
With a parameter override of: <none> (although this box is actually automatically greyed out for this one)
Once this is done, compile your map and choose a team. Then run the centre of your map where trigger1 is. As a member of the blue team, you should hear the announcer shout "Success!", and for red team "Failure!"
This will happen every time a player runs through the trigger, and you should be able to adapt this little tutorial to whatever you want to play some sounds for specific teams.
It's similar to what kimangroo has posted I suppose, but I hope it helps.
