Hi sal2,
As stated earlier you could most certainly use the USB device to generate sound, but that would be at a max update rate of 150 Hz. While according to Nyquist theorem you could get frequency information for signals below 75 Hz, you may notice that the quality of the data in that spectrum to be very low due to having so few samples.
While technically possible to produce you really should look for a device with a faster Analog Output update rate. I would look for a device that supports Analog Output at least 10x the maximum frequency that you want the user to hear. Some great, yet lower cost products, would be the
M-Series line of products. They would give you the performance that is really needed in the type of application that you are talking about.
If you still want to use the USB Device, then you would need to use code similar to that found in the example
Gen Mult Volt Updates-SW Timed (Found here:
C:\Program Files\National Instruments\LabVIEW 7.1\examples\daqmxbase\Dynamic\ao).
Best of luck getting your system together,