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The issue is larger than just barking, so I'll share some of the things I know from my own experience. Sometimes older dogs with health issues can exhibit strange behavior - like barking for no readily discernible reason. If you suspect arthritis is an issue, that is a very painful condition, and some dogs just bark a lot when they're hurting. Ask your vet for an anti-inflammatory med like rimadyl or dermaxx. If you can treat the pain, some of the barking may recede.
Since he has cataracts and can't see nearly as well, he is relying even more on his nose to tell him what is going on in the world. Consequently, he has to make sure that YOU know what is going on in the world. He relays the message via barking. That sharp, high-pitched bark sounds like he's trying to raise an alarm. If he can't see well any longer, there's a whole lot more things to be alarmed about now.