Ken Stone: Digital Audio in Decibels & Final Cut Pro’s Audio Peak Detection Tool
While Apple and Sony (for mini-DV) set the standard at -12 dB, -20 dB reference is the standard in most professional digital audio for picture, including Digital Betacam and HD. Some might say that peaking at -12 dB is still too hot and they use -20 dB as their reference.
Don’t be afraid of lower volumes. This is digital audio and lower volume audio is quite clean.