I like the Idea of a horn, and you could have 3-4 different crafting options for pitch/size. For example, small light horns that can travel short distances and have a high pitch, vs large heavy horns that have a low pitch and can travel long distances. The horns should also be able to play various notes (eg 5 notes) in a 2-4 second horn blast that the player chooses as he/she blows the horn. That way they could have custom signals for the various messages that they want to send (we are under attack, i need help/i found something, visitors approaching, marco/polo, attack, retreat...)
As far as the value of these in a world with teamspeak/ventrilo: You cant always expect that your guild members will be on TS/V or in your channel. And other guild members will never be in your TS/V. If you walk up to a closed city gate, it would be nice to have a loud way to request entry. Also TS/V doesn't tell you direction, where an in-game generated noise would be able to be found by following the direction of the noise.