Join the Dog Training Advice and SUpport group on FB - its the one with over 300K members. Read the guides as they may answer the question and if they don't, then ask (Stating which guides you read so the admins know what you've already gone over).
Barking in any dog may be down to habit, fear, anxiety, boredom and prior learning.
Add in an older dog, there could be a level of cognitive decline.
I would not attempt to 'train' this out of the dog by making rewards contigent on good behaviour because your dog does not know what it is you want, because barking is self reinforcing and this will simply cause confusion for the dog, and frustration for you.
Food is food, dogs don't grasp any difference between 'human' food and 'dog' food, it is all food. Only humans are weird like this. Dogs have foods they like, love and hate just like we do. Use one that is convenient to you that your dog actually likes (attempting to reward anyone with something they don't like is beyond stupid, though I find people doing this with dogs fairly often).
I would try some basic classical conditioning pairing and any every noise in and out of the house with a treat, to try to alter the dogs emotional response. This needs to be consistent and is pretty intense at first, its just the start though its not the full deal, later we move to fading out some of the treats and mixing in some simple 'can you sit first, then treat'...
I'd also look to reduce as much stress/frustration as possible so they can unwind, relax and are in a better position to learn. High stress levels are going to inhibit learning.
Sleep - aim for the best possible sleep, just like us, dogs who do not sleep well are easily stressed, cope far less well with life and find it hard to learn.