Long after most other memories have become inaccessible to a person in a more advanced stage of Alzheimer's disease or some other progressive dementia, that person might remember songs from his or her past and even sing along with them. People often report being surprised when their mother or father who doesn't remember who they are sings a familiar song without missing a word.
It's not just the stimulation caused by the memory of the song that is gratifying and therapeutic, but reminiscences are also conjured. With those reminiscences, emotions that became associated with the songs long ago are brought out and relived. It's common for a song to elicit smiles, laughter, and even tears.