@Woodburn,
Although I'm not familiar with how your Sonos system works, after reading your post, I'm a little confused. Right now it sounds like you keep ALL of your music files (the entirety of your library) on an external disk that you also use as a Time Machine backup? You also have a smaller subset of your files stored on your MBAir? It sounds like you've got multiple copies of your actual music files in different physical storage locations (some on your MBAir, some on a Time Machine drive).
As glynor said earlier in the thread, MC doesn't care WHERE your actual music files are, and MC won't move your physical files unless you tell it to. In MC-terminology, the word 'library' specifically refers to the database that MC builds. That 'library' is located (by default) local to your machine, but the music files that the library refers to can be anywhere: an internal drive, an external drive, a networked location, etc.
When MC imports your files, you are simply telling it to "Look here for music files"; "Look over there for music files"; "Look in this other place"; etc. When MC plays back, or you tag the files, it will play the file from its file location--wherever that may be.
So, to try to answer your questions, your MC 'Library' will be located on the machine you run MC on. Your music files will stay wherever you currently have them. If you want MC to only import files from your MBAir, simply import from the local hard drive. If you want MC to import files from your Time Machine backup, import them from that drive.
Once imported, MC will read and write from the files, without messing with your filing system or your iTunes database....
'Zat help?
brad