Can`t change Audio Driver Bit Depth

Hi Sonar friends,
I was trying to change the audio driver bit depth but it`s impossible because the function is locked/dimmed on 24 besides the selected 64 double precision engine.
My audio interface is M Audio Fast Track Pro and I am running Win 7/64 and my Sonar is Platinum.
Any help?
 
Thank you

Looks like you may be up against a limitation of the device:
Configurations
At 16-bit operation, the Fast Track Pro operates as a four-in, four-out device at a maximum sample rate of 48kHz, with analog and digital
inputs available:
< Analog Input 1 & 2 and S/PDIF input
< Analog Output 1 & 2 (available on both TRS and unbalanced Outputs 1 and 2)
< S/PDIF Output (mirrored at unbalanced Outputs 3 and 4)
At 24-bit operation, the Fast Track Pro operates as a two-in, four-out device or a four-in, two-out device at a maximum sample rate of
48kHz, with analog OR digital inputs available:
< Analog In 1 and 2 or S/PDIF
< Analog Out 1 & 2 (available on both TRS and unbalanced Outputs 1 and 2)
< S/PDIF Out (mirrored at unbalanced Outputs 3 and 4)
Or
< Analog in 1 and 2
< S/PDIF In 1and 2
< Outputs 1 and 2 or 3 and 4
At 24-bit operation, the Fast Track Pro can also operate as a two-in or two-out device at sample rates of 88.2 and 96kHz. Analog or
S/PDIF can be selected for input or output.

I was wrong, bit depth is not in the Mix Module, it is displayed in the Transport Module. Both modules are part of the Control Bar.