playback volume low

This is a very good question. Here are links to the three for easy reference:
Amplify
Normalize
Loudness Normalization

They all do pretty much the same thing. Amplify by default will amplify to maximum permissible without distortion, but it assumes there is no DC offset introduced by the recording equipment (you can’t hear DC offset). See also: Amplify and Normalize

However, one extremely loud noise or clap or gun shot in a recording can force the remainder to an unacceptable low value. More commonly, the absence of dynamics could force a recording to be unacceptably loud (think commercial). “The perception of loudness is related to sound pressure level (SPL), frequency content and duration of a sound.” - See Loudness