Is it possible to have CS:GO lower the volume instead of mute it when alt-tabbed?
No, not via the help of default configuration options. That's not an option provided.
However, you can try something else. You can turn on the option to "Play audio while game in background" so that it'll kepe playing when you (ALT+TAB) out. Then you could try using macros if you have a programmable keyboard such as Logitech G510, and assign multiple actions such as VolumeDown+(ALT+TAB) for one programmable key and VolumeUp+(ALT+TAB) for another one, and use these programmed keys instead of (ALT+TAB) combo.
Or if you don't have a programmable keyboard, you could try using a utility such as AutoHotKey and pick some weird key combination like (AltGr+M).
Neither of these is the perfect solution, but at least they'll do the trick.