I think it was key [HKEY_CURRENT_USER\ControlPanel\SoundCategories\InCall] and value InitVol. I now use "Profiles for Windows Mobile", and it has an option to set this with each profile, so I dont have to change the reg key anymore. I am not 100% sure about this key because it did not change when I changed my sound profile. However before I started using this app I had set it to max vol initially through the registry and I'm almost positive that that's the key.

Let me know if that works