Hi @mkladywendy ,
There may be a problem in the wiring harness connections between the steering switches and the radio, possibly near the steering column, maybe harness connector M88 (see horn information below).
Here’s a link to the 2010 Nissan Altima – Audio, Visual & Navigation System section of the repair manual. (you didn’t state the year model or the vehicle type in your question so hopefully this is the same for your year model).
It describes in detail the procedure for testing the functions of the audio system. This includes the steering wheel switches. Start on p.AV-34
With regards to the horn connection from the steering wheel, the horn button is wired to the IPDM E/R (Integrated Power Distribution Module - Engine room) CPU. When the button is operated it is the electronics in the IPDM that operates the horn.
Here’s an image showing the horn circuit from the steering wheel. You will notice that it also goes through harness connector M88 as does the steering wheel switches so perhaps it may be worthwhile checking this connector first. It may have come loose.
(click on image to enlarge for better viewing)