When I cast a spell does the verbal component has to be clearly audible?

@JeremyECrawford when I cast a spell does the verbal component has to be clearly audible or can I "whisper" the spell?

— _Blackstorm (@_Blackstorm) September 25, 2015

The verbal component of a spell must be audible to work. How loud is audible? That's up to the DM. #DnD https://t.co/NxFu0NSHuo

— Jeremy Crawford (@JeremyECrawford) October 6, 2015