The normalize command scales the amplitude of an audio clip.
To normalize a sinusoidal sound to −6 dB:
| t:=soundsec(3):; L,R:=sin(2*pi*440*t),sin(2*pi*445*t):; s:=normalize(createwav([L,R]),-6):; | 
Indeed, since about computes the level of audio:
| about(s) | 
Bit depth: 16
 Channels: 2 (stereo)
 Sample rate: 44100
 Length: 132300 samples
 Duration: 3 seconds
 Level: -6.00048 dB