I have the 18i20 and have what seems to me a strange issue. When I use Sonar and any other media source there is no sound from the second source. Such as having Sonar open and say, Guitar Pro. Guitar Pro will not play. Same goes for any media player, youtube, etc. I have to close Sonar to play these other sources.
 
I've had other recording interfaces that didn't do that. It played whatever I opened regardless of what else was opened.
 
Anyone know how to fix this with Focusrite?

You need to check share driver in preferences. Also if your using 48 in Sonar you'll have to close it to use other apps which all use 44.1 

Yup, it's mostly a sample rate issue.  Your interface can only support one rate at a time.  If Sonar switches your interface to something not supported by Windows or the other apps, that's what happens.

It's probably not an interface issue, more likely it's a driver issue. Try switching to WASAPI for SONAR and see if that doesn't take care of the problem. I have essentially the same interface (the Firewire version vs. USB) and don't have this issue.

Cactus Music
You need to check share driver in preferences. Also if your using 48 in Sonar you'll have to close it to use other apps which all use 44.1 

This is not true. All that needs to happen is make sure the Windows sample rate is set the same as SONAR.
Here is how to configure the default playback device in Windows:

• In the Windows Control Panel, open the Sound tool.
• In the playback tab, click on the default device.
• Click the properties button
• Click the advanced tab in the Properties dialog to set the default sample rate and bit depth.

Thanks for the replies. Matching the bit rate was the solution. However, it seems it must 44.1 in order to make it work. Sonar was set to 48 and windows at 44.1. I had to drop Sonar down for it work. setting both to 48 was a no go. Windows said it wasn't supported.
 
Seems odd. Is that right or is something else possibly going on?

If you're running two simultaneous applications... both using ASIO, you need to make sure the app running in the background releases the ASIO driver.  Otherwise, you'll hear no sound when moving to the second app.

I have not had any more problems since setting everything on my system to 44.1.
 
A while back I tried setting both Windows and Sonar to 48, but something here did not like Windows running at that rate.  Don't recall now exactly what it was, but I ended up switching it all back.  Might be worth a try...
 
I have heard others make comments about dedicating their audio interface to the DAW at a higher sample rate, and then using the on-board sound chip for Windows at the lower rate.  Need another set of speakers for Windows audio that way though.

Jim Roseberry
If you're running two simultaneous applications... both using ASIO, you need to make sure the app running in the background releases the ASIO driver.  Otherwise, you'll hear no sound when moving to the second app.

Right on, except he also mentioned media player, YouTube, etc, and those are not ASIO applications. 
 
I have found one trick around the one ASIO at a time limit, is to set the second app to use WASAPI or whatever.

I just did an experiment to see if I could play a DVD and a YouTube video at the same time. To my surprise, I could. The Focusrite showed its sample rate as 44.1 KHz (my normal rate for everything, and the Windows default). Given that the DVD audio is 48 KHz, that tells me Windows is quietly doing a sample rate conversion behind the scenes. I'm guessing that if I had designated 48 KHz as the default, Windows would have done the conversion for the YouTube video instead of the DVD and all would still be well.
 
Note that Windows explicitly labels this setting as "the sample rate and bit depth to be used when running in shared mode", an acknowledgement of the fact that any interface can only handle one SR and bit depth at a time.
 
So back to the OP's question: why doesn't this always work? My guess is it's because of ASIO, which bypasses much of the Windows audio subsystem and therefore cannot benefit from this automatic SRC.