[Edit: James posted much of the same information while I was composing this reply.]

Check fuse 2 to make sure it's not corroded by spinning it a little, or pull the fuse to make sure the ends aren't worn through from spinning.

Test the horns individually by disconnecting the other. I'm guessing one might be gone.

The contacts rarely corrode, but definitely clean them up to see if that helps. If that fails, use an ohmmeter on the black wire to check that you're getting a reasonably decent ground when the horn button is depressed. Then, with the ignition on, verify 12V on the yellow wire.

You can also try supplying your own 12V and ground to the horns individually. They will normally be noticeably louder when directly powered like that.

Note that chassis ground is supplied through the horn button (not by the horn mounting bracket). Although fairly rare, you may need to clean up the copper horn ring at the base of the steering wheel or replace a worn horn ring contact button.

Wear ear plugs or muffs as needed.

https://www.youtube.com/embed/KaK7T2x-jFY?si=r5wI4EIN6w1XnqGi SE Volvo 240 Horn Diagnosis and Repair video
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

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.

Hi _?_,

Check for bad horn connections, get new bullet pigtails here:

https://www.prancingmoose.com/blackvinyl.html#pigtailfemale

Fill the female bullet pins with Penetrox-A to deter corrosion long term.

Good luck, Bill