The following steps have to be followed in order to get an FX2LP audio device to appear in the Windows Volume control:
Specify in the feature unit descriptor which features of the audio device need to be controlled, for e.g. volume, mute, tone, AGC etc.
Modify firmware to respond to the class specific requests from the host.
Feature unit control requests are used by the host to get and set the feature unit controls. For each control that has to be implemented, the host will send a series of feature unit control requests after enumeration. The firmware has to be modified to respond appropriately to these requests.
For example, if it is specified in the feature unit descriptor that volume control is needed, after enumeration, the host will send requests to get the current, minimum and maximum values of the device?s volume. Code should be added to the setupcommand() function in fw.c so that the device responds to these requests. If the values returned are within appropriate limits, then the device name will get listed in the volume control panel. For volume control, the current, minimum and maximum attributes can rangefrom +127.9961 dB (0x7FFF) down to -127.9961 dB (0x8001).
To find more information on feature unit descriptors and requests please go through the document ?USB Device Class Specification for Audio Devices' from the following link: http://www.usb.org/developers/docs/devclass_docs/audio10.pdf