Apple Compressor 3 - Manual (Page 248)

Sound Sample Size
The sound sample size determines the dynamic range of the sound. 8-bit sound provides
for 256 possible values, whereas 16-bit sound allows for more than 65,000 possible values.
Choose 16-bit sound for music that has both soft and loud sections, such as orchestral
music. For spoken voice or music that has a more or less constant volume level, choosing
8-bit sound can still yield good results.
If you need to shrink your media file, you can reduce the sample size from 16 bits to 8 bits.
This cuts the file size in half but also degrades the audio quality.
248 Chapter 20 Creating QuickTime Movie Output Files