I have an Echo Audio Layla 3G audio/MIDI interface and it works beautifully - unless I happen to open up iTunes. iTunes will work fine but it seems to grab the audio card away from anything else. If I open up SONAR or Sound Forge, I get messages that the audio card is unavailable.
 
I looked in iTunes' Preferences -> Playback tab, there doesn't seem to be much I can do. It's only got two choices under Play Audio Using - Direct Sound and Windows Audio Session. SONAR and Sound Forge use the Echo's ASIO drivers, so maybe that is an issue.
 
It's not really that big of a deal, I can just not launch iTunes when I'm working on any recording projects. Back when I had a Delta 66 on Windows XP, I used to use Sound Forge sometimes to record off the audio output of iTunes so I could make an MP3 that didn't have copy protection, but I've found that I can work around that by opening an iTunes mp4 file and exporting it as a WAV file that I can then convert to MP3. 
 
Any ideas?

ASIO is designed to be exclusive and you should be using it in Sonar. Under preferences you can tell Sonar to share drivers with other programs but I don't recommend this. Many people use their internal sound chips for this purpose rather than use their interface, I would move Source Forge software away from ASIO otherwise (although not ideal).
 
Also please have a go at updating your soundcard drivers, probably won't make any difference but worth a go.

Thanks, Alex. I have already installed the latest available driver for the Echo.
 
I can probably live with it like this, I almost never feel the need to have Sound Forge open while I'm using SONAR.

It's gotta be an Apple thing or the sound driver....

I can play Sonar (ASIO), Sound Forge (ASIO), and Windows System sound (Browser, regular apps) all at the same time - though there's not much reason to do so. (Rather messy sounding..)
 
Sonar does not have 'share driver' enabled, Sound Forge is using ASIO also, and Windows is set to Wave Audio with allow exclusive use and give exclusive application priority checked. I'm running Win 7 x64, and using a MOTU MKIII ultralite. MOTU has one of the best drivers around, they support ASIO, WaveRT, and regular Wave audio - simultaneously.