Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With a Beagle, watch for moments that do not look like them. A seizure can begin as staring into space, twitching or confusion, and their health record puts the typical start between six months and two years. A first seizure, one that lasts more than two minutes, or more than one in a day is an emergency: contact your vet now. So is bloat (GDV, a twisted stomach): a swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling. It can happen in any dog, though it is most common in large, deep-chested breeds. A Beagle who has gone quiet, slow and heavy may have an underactive thyroid. One who cries when you lift them, or will not jump up any more, may have a sore back. Film it on your phone and show your vet.
The neighbours will get used to the singing. If your Beagle stops joining in, call your vet.