Have you checked the FAQ? - much of the information in the FAQ pertains to ALL cars (not even just Volvos!).

Most horns work when they are grounded at the steering wheel. They always have 12 volts going to them and sound when you ground the circuit at the steering wheel. This is why horns can sound continusly sometimes in accidents that do not damage the steering wheel.

Disconnect each horn and use a meter to see which wire is hot and if it has most of 12 volts.

Reconnect the hot wire and ground the other wire - the horn should sound. Do this with each horn. This checks the horns.

If each horn sounds good, measure the resistance to ground on the ground wire for each horn when your assistant presses the horn ring. This checks the rest of the circuit. Do not mix the wires up when doing this test as you can ruin the meter movement by putting voltage to an Ohmmeter.

I would not be surprised if the horns have died of old age. There are many nice aftermarket horns, particularly if you might like a different sound.