What are the ranges of values for each audio feature/descriptor?
Unfortunately, the original standard is not available for free. Furthermore, freely available information on the MPEG-7 descriptors seems to be scarce. Therefore, my recommendation is to get [1] which contains in-depth information about all descriptors, including their computation. Once you understand what each descriptor is supposed to describe, and how it is computed, you'll get an idea about its range of values.

The following resources may also be of interest:


An online computation of a number of MPEG-7 audio descriptors for a wave file


References

[1] H.-G. Kim, N. Moreau, and T. Sikora, MPEG-7 audio and beyond: audio content indexing and retrieval. Chichester, West Sussex, England; Hoboken, NJ, USA: J. Wiley, 2005.