Above is a ceiling to wall assembly illustrating how a ceiling and the wall's drywall layers should intersect to provide a good seal. Note that we want solid drywall to drywall contact.

Don’t try and leave a gap. The illustration shows a gap that is 1/32 of an inch. Most drywall installers do not hang drywall to this tolerance, but the point is you are not trying to leave a gap on purpose. After drywall is hung, seal the seam tight with acoustical sealant.

Summary:

Applying acoustical sealant reduces the risk of seal failure, as the remaining gap is small and easy to maintain over time. We are also maintaining mass in the gap area but sealing it tight like a fish tank. Nice and tight, be have now removed the possibility of a sound leak.