I have two Focusrite interfaces. One is a Saffire Pro 24 (connected via firewire) and a Scarlett 2i2 (connected via USB)
I have been using the Saffire Pro 24 for quite awhile (along with the Saffire software Mixer) without any problems.
But recently I've been getting issues with the Pro 24.  For example, after I closed out a session of Sonar, I'm unable to hear any streaming audio or any audio files on my computer.  If I reboot, then I sometimes can hear them.  There are other issues the interface was causing, but I won't go into all those details right now.  I used to be able to listen to streaming music from the Internet while playing along in Sonar at the same time.  But now it's only one or the other at a time.

So I switched back to my older Scarlett 2i2,  I downloaded the latest drivers and files.  It seems to work fine on both streaming music and audio files on my hard drive.  It even works in Sonar.  But whenever I open Sonar up,  I get the message that there is no Output Device connected.  And when I go to MIDI Devices, there is no Focusrite device listed there.  The only outputs it shows are, Microsoft GS Wavetable synth, Oxygen 88 (my midi keyboard, input only), and Game Controller 1.  There is no Scarlett 2i2

If I close the preferences box, and open up a project, I DO hear the audio coming out of the speakers, and headphones.  But I'm not sure how or why.   None of the three listed output devices are checked, and yet I'm still getting audio.

I am able to record with a microphone using the Scarlet 2i2, so I know the USB connection is good.  It's just that in the preferences there is no listing for that interface.
I'd really like to go back to the Pro 24 firewire interface. (I read that Firewire connections are better than USB)  But now it seems both aren't working as they are supposed to.  There seems to be some conflict with Sonar and all the other audio I use on my computer.

Any suggestions or ideas?  Thanks!

I reinstalled the drivers for the Sapphire Pro 24 and reconnected it.  It seems to be working properly (for now).
So I guess I'll stick with it until any problems start popping up again.

But if anyone has any ideas as to why Sonar won't recognize the Scarlett 2i2, please let me know.

SONAR is constrained by what Windows is controlling. If you check your sound cards in Windows ("Change Sound Card Settings") by highlighting them (one by one) and then "Properties." In the Advanced tab, I have both "Exclusive Mode" options unchecked. This will allow applications (who have the ability to assign exclusive modes) to function properly, but I leave those unchecked as well. I also leave my onboard Realtek as the "Default device" so instances like a browser streaming audio does not "lock" the bit rate on it... that feature is is "first come, first served" so if an app locks the Saffire at a given bit rate/bit depth, it will not be able to stream other audio.
 
If you use it the Saffire for general purposes, I would also set the "Default format" on the Advanced tab to what you normally run SONAR at to avert potential conflicts (since Windows will pretty much be using it (and lock it) before SONAR can see it).
 
Windows can potentially reset those properties on an update, so they may require re-visiting from time to time.

You can try deleting AUD.ini and let sonar rebuild it.
Its possible because you are using 2 hardware devices (the 24 and the 212) from the same vendor (Focusrite) that something written in the driver or Focusrite software itself is making Sonar believe one of them is missing.
 
Now as for why your onboard soundcard is being held hostage somewhere, I have no idea.

I'm currently using a Safire Pro 40 on win 10. I haven't really had any issues with it, but I noticed a while ago that it stopped being so good at letting go of Sonar when switching to other sources that are at a different sample rate. Most of my projects in Sonar are at 48k where as most stuff in windows is 44.1k.

That's exactly the issue, If you want to use your interface for all sound on your computer then 48 is going to toss a monkey wrench in there. I always use my interface for all audio but I'm usig 44.1.
 
Note, of course the 2i2 didn't show as a midi device, it does not have midi ports. 
 
Instead of playing along with songs while steaming why don't you download the song and drop it into Sonar. 
I use a app called savefrom net  http://en.savefrom.net/1-...ownload-youtube-video/

Re Mettelus' post, with my Focusrite 18i20 2nd gen, exiting Sonar could change my listening sources until I created profiles with Focusrite mixer working in tandem with Windows mixer re realtek HD, others, and I listen to YouTube, internet radio, etc simultaneously no problems.

Thanks for the tips, I appreciate it.  For some reason the Saffire Mix Control software always baffled me, especially in combination with Windows audio settings, it's like a big jigsaw puzzle, where all the settings have to line up just right for it to work.   But from what you guys have told me, I think I have a better understanding now.

One of the oddities that I didn't mention (when things were REALLY screwed up), I'd go to a streaming audio site, and select a song, and the online player would quickly run through all the songs, like a half a second for each song, and keep scrolling and scrolling.   I clicked on my songs on SoundClick (where I have 80+ songs) and it kept quickly scrolling through all the songs, and repeating when it got to the end.   And SoundClick bases it's song chart positions on how many clicks your song gets, so through the technical screw-up of my audio settings, my songs got about 20 hits per song that day.  It shot many of my songs up to the top 10 spots on the charts.  I had like 500 song hits that day.   It was crazy.  (I only tried it twice, but if I wanted to, I could've kept it going and artificially pushed all my songs to the #1 spot on all their charts.  But my morals and ethics got the best of me....I even thought of emailing them to tell them what happened)

I used to have a similar issue, suddenly there'd be no sound from the interface (not a SONAR issue). The problem seems to have since gone away on its own, so I never figured out what was causing it. But here's a tip for bringing sound back if it happens again: rather than rebooting, just open MixControl and change the buffer size. That forces the interface to re-initialize itself, and for me has always brought it back to life.