Hmm, I have an Inspiron 6000 and I never used these keys when I had XP. However, these function keys (Fn/F3, Fn/F10) require Dell QuickSet, no matter what version of Windows you have, while other function keys (like Fn/F2) work via the bios. And yes, you can use QuickSet under Windows 7, although if you search the Internet you'll find nothing but people who say it cannot be done.
Here's how to install QuickSet in Windows 7. Go to support.dell.com, enter your service tag, and download the installation program for QuickSet. (The one I downloaded saved as R130907.EXE, version 7.1.10.) Right click on the file, select Properties, then Compatibility tab, then select "Change settings for all users" and enter your administrator password, and then select "Run this program in compatibility mode for: Windows XP (Service Pack 2)", although I did it with Service Pack 3 mode. Now right click the file, run as administrator, follow through the install, and reboot when it asks.
After you do this, let me know if QuickSet displays the volume properly when you press the front volume buttons. For me, QuickSet only displays max volume, no matter what the volume actually is.