Sounds like it's suffering with separation anxiety. I would get a hold of a behavior trainer and see what they might be able to do for you.

 Make sure there is some toys to play with or give your pup a dog bone.

 With behavior problems like separation anxiety it is best to work with your veterinarian and a behaviorist (trainer).  If this is a new behavior your veterinarian will help screen for an underlying medical cause for this anxiety.  Behavioral issues are solved through behavior modification and anti-anxiety medication.  Often initially they are used together and then we try to wean dogs off medication.  Your veterinarian will do a bloodwork screening to make sure your dog can tolerate medication then prescribe an appropriate behavior drug. 
Do not use a muzzle.  The only humane muzzle to leave on a dog unattended is a basket muzzle.  This will not stop your dog from barking.  Cloth muzzles are much tighter, they can still bark with this muzzle on, and are not safe to leave on.  They cannot pant adequately-which is how dogs cool off, and it can restrict breathing.

 You might want to try a muzzle.they are safe to use and it will keep him from barking. you can find them in pet stores, in different sizes. Make sure you get the right size. Good luck with your barking dog.