Real audio signals leave many coding levels unused and waste bits.

Evidence

Consider the five-level audio signal shown in column 6 of Table 7.9. At the moment, we are using 3 bits of information to transmit an information source that only has 1.7 bits of information. How can we assign code words to the symbols such that the average data rate is closer to 1.7 bits? First, we can recognize that three of the symbols are not used and hence can be ignored. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2636