Hi,
 
I'm using an alesis multimix 4 usb to record audio into sonarX2 in windows 7, but there is a usb hiss. I had the same issue with with my logic pro setup and found the usb hiss came from using the mixer to input and output audio, which was easily resolved by using the internal speakers as the output and just using the mixer as an audio in. I tried to resolve the problem in the same way on Sonar but it didn't work. 
 
Any help would be much appreciated...
 
Best

Where are you hearing this hiss?
 
Is it on your recordings or when Sonar is idle?
 
The Alesis is only a 16 bit interface, so it's possible you're not cranking the input gain enough to rise above the noise floor.
 
This  might give you some more information, as you don't seem to be the only one with the problem

FWIW, The issue may be ground related... or the result of using a poor quality USB cable... but the issue isn't USB itself.
ie: I've got a RME Babyface (bus-powered) and the noise-floor measures -114dB

I had a notebook once that hummed like crazy on USB.  Seems like I used one of those Ebtech things and that fixed it.

To fix high pitch noise try disabling the USB selective suspend setting under USB settings.

Are you sure the "hiss" is caused by USB? I think USB (i.e. digital noise) is more like a whine. Hiss, or white noise, would be caused by various factors inside the interface's amps, circuits, components, etc. You can always add a gate plugin to your audio tracks with inputs from the interfaces. The gates would have a threshold of a very low value to only block out the hiss/noise-floor.

I'm sure what your are hearing is a ground problem or a cable problem (not using shielded cables for your studio monitors).
 
If your using a Laptop, try using the battery power to see if it stops. If it doesnt', get the proper cables for your studio monitors.
 
If that doesn't work, you have a ground problem and you may need to call an electrician. Or your elesis is crapping out. Start ruling these things out.
 
CJ

I don't think he's coming back