On the CD, the sounds are present, it is just the Content Project file isn't. The zip file in the errata page has the missing Content Project file and an updated Game1.cs file. So you should copy the filesfrom Chapter 7 from your CD onto your harddrive. Then you should unzip the Chapter 7 Fix.zip file from the errata page overtop of the existing files. This will give you what you need to actually compile the Chapter 7 code.
To your point, Chapter 15 does include the Content project that has everything that Chapter 7 has plus some other items. You could just copy the Sounds folder from Chapter 15 if you would like.
Hopefully this helps,