I think that some voice over would be great for notifying the team when the flag has been taken or captured. I would especially like a notification when the flag is captured, i.e. “Our flag has been captured again!” I will sometimes lose track of the score because the only notification is a little chat message, and I don’t like having to pop open the score screen to know what’s going on.
The only problem I see with notifying the team when a flag is picked up/dropped is that flags can change hands relatively quickly at times and you end up with either voices cutting each other off, over lapping, or just getting really behind what is actually happening. I think that, as Germafias suggested, a couple of distinctive sounds indicating friendly/enemy flag pickup/drop would be helpful.