Danny with diluted Siren blood(it skips a generation in non purebloods so nobody knows of Danny's Siren Blood, Danny only finds out after becoming a ghost), he sings as an instinctual need, his song can not hypnotize or ensnare due to his diluted blood but it has other effects, in humans his song calms their minds(soothes anxiety and over thinking and allows them to feel calm), for ghosts, his song can help restless spirits and shades(not ghosts/revenants, liminals ect) pass on to the afterlife as it temporarily calms their restlessness down and helps them find peace, as an Adult Danny travels to very death heavy areas and sings to help spirits and shades move on.
Note:
-Danny doesn't have a siren/mermaid form, his blood is too diluted for that, BUT his Ghost Form has some Siren-Esc elements, like webbed and clawed hands, sharp teeth, and his ghost tail looks like a Siren's tail but he can still have legs in ghost form, this is part of why nobody connected Danny to Phantom.
-singing is an instinctual need, even before he realised he was part Siren, Danny needed to sing, mostly he sang as he worked or did chores around the house. Everyone just chalked up their calm feelings when Danny sang as being because he had a beautiful voice.