DirectSound: how to change the input volume of a microphone? (I'm not far from solution I think)

Question

Hi,

I have some questions about Directsound and windows mixers.
My goal is to enumerate all microphones and be able to change the input volume of each one.
I think i'm not far from the solution, but I don't find what is wrong in my code.
Here is what I have done:
- I enumerate all input devices and get a GUID for each one
- I use a method found on a topic to get the mixer id corresponding to a directsound guid using this method   (but I'm not sure if it works)
- Then I get the id corresponding to the control in the mixer
- Then I can modify the volume

Here is the code: a vs2008 project

To test, I have connected two microphones usb + the line-in microphone, and I visually check what sliders are moving. But unfortunately it's not the good one ...
here is a screenshot of all my mixers opened in windows xp.

Have you an idea of what I am doing wrong? Is there an easiest solution?

bonus question: do you know if there is a way to know if a microphone is connected or not in Line-in, using Directsound? Because the Line-in is always detected as connected even if no microphone is connected.