I just bought a USB mixer. Not because it was USB, but because my other one was flaky. I was told about this USB feature at the store, and don't know how to get Sonar to see it. I've plugged the cable in, and Windows used that ASIO4ALL driver, but when I started Sonar, I expected it to detect a new input, and when it didn't, I then expected a new input option to be available for a track, but I don't see anything. I poked around the Properties window, but am at a loss. Also rebooted the PC to see if that would help.
 
Like I said, I didn't buy this because it was USB, and am currently fine using my regular M-Audio sound card, but I was told the USB connection might reduce noise, plus I'm just curious if I can use it.

It would sure be usefull if you told us what the brand and model was??
My band just bought a Yamaha MG XU 16 mixer with USB ( stereo) connectivity. It comes with a Steinberg ASIO driver. 
I didn't buy it for recording but it actually seems to be as good as any basic audio interface and I would think it's more or less equal to the Steinberg UR22. 
Best is it works side by side with my Tascam us1641 in WDM mode. 
I'm recording my band live into Sonar and on breaks I playback tunes on Media Player. 
While media player is running I have kept Sonar open and save the set I just recorded and open a new project ready for the next set. So no issues with drivers at all. 

Possibly your installation of ASIO4ALL is masking the ASIO driver of the new unit. What model is your new unit?
 
Meanwhile, if you already have another device configured to use ASIO4ALL, you may need to go into the config dialog of ASIO4ALL to choose your new device. Sonar won't see that choice.

I have a cheapo Behringer 1204USB that actually works pretty good as an interface, except for the stereo only USB side.  Oh, and the overhead in the pres isn't very good either.
 
Still, for recording a track or two at a time it isn't bad at all.

Sorry everyone, duhh... It's a Behringer Xenyx Q802
(who comes up with these names?)

if you have the M-audio Delta installed and are running ASIO driver mode then you can't use the Behringer USB mixer also as a direct input to sonar because ASIO only allows the use of ONE device at a time.  and you wouldn't want to use the delta and the behringer at the same time anyway because they have 2 different clocks and your recordings would have timing/drift problems.
 
also, you shouldn't use ASIO4ALL if you're using the Delta anyway - use the native M-audio ASIO drivers instead.  ASIO4ALL is probably needed for the berhinger USB, but as I said, you can't run both at the same time anyway.
 
if you installed ASIO4ALL because that's what Berhinger said to do for the USB mixer, then you need to make a choice of whether you want to run the delta or the behringer.  if you want to run the behringer, you'll need to disable the delta in your windows hardware settings.  then the usb mixer would work under ASIO4ALL (which I believe that's what berhinger usually says to use for their USB mixers).
 

I don't really care which one I use. I only need one stereo input anyway. I really just wanted to try out the USB to see if it was better or easier or anything. 
 
I tried plugging in the USB and Sonar popped up a message saying there was a new input and asking me if I should install it. I said Yes, but in my track inputs and the Properties page, things look the same. The Windows hardware page indicates a new input, namely USB Audio CODEC, which I set as the default. But nothing in Sonar says anything about USB. The only inputs still say M-Audio Delta ASIO etc.
 
I was just about to unplug the RCA from the M-Audio card to see if it is maybe recording into USB anyway, but I realized there's another complication I'm confused about:
 
As mentioned, my sound card only has the single stereo in, which is really fine for my needs. However, it means I need to use the Effects send to record anything to the computer while still hearing everything else. So my mixer's effects send goes to the PC input on the sound card. (The sound card's output goes to the tape in on the mixer for monitor purposes.) When I want to record a vocal, or bounce down a Midi track, I just bring up the effect volume knob for that channel and make sure everything else is 0. Works fine. (If I need to record a stereo Midi track, or my digital piano, I repatch a bit to record the stereo, since the FX send is mono).  So if I switch to the USB, don't I lose the ability to record into the PC on one channel and simultaneously monitor the others? 
 

Which M-Audio Delta are you using? I used to have a Delta 44 PCI card interface that gave low enough round-trip latency for me to monitor the inputs using Sonar's audio track echo function. So unless I needed to record more than four inputs at a time there was no need to set up any hardware monitoring other than connecting two of the Delta outputs to the nearfields.

As for Sonar and the Behringer, have you tried selecting something other than ASIO in Sonar's driver preferences? You might find it shows up then. Looks like for ASIO using it you're stuck with ASIO4ALL which can be tempremental to put it mildly. It might be worth trying the WDM driver if Windows makes one available for it, I think the mixer's supposed to be class-compliant in Windows (and Core Audio compliant on Macs).

As for "Xenyx", I'm quite sure the name has nothing whatsoever to do with Mackie's released-earlier mid-range mixers being called "Onyx". All purely a coincidence.

jkoseattle
Sorry everyone, duhh... It's a Behringer Xenyx Q802
(who comes up with these names?)

Take it back and get the 802 without USB. 

Looks like Behringer has stopped providing their own drivers.  I got real drivers with my 1204USB.  The readme says the drivers support a bunch of different hardware.
 
If you want to try these drivers, PM me and I'll put them up somewhere for you to download.
 
Some people have great luck with ASIO4ALL, others don't.  I'm in the don't group.  ASIO4ALL isn't a real ASIO driver.  It's layer on top of WDM to make it look like an ASIO device.