[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