Hello,
 
I have a Lexicon Lambda audio interface and a sound card attached to my computer.  My musical productions output the Lexicon Lambda.  Most of the time, this is preferred.  Sometimes, however, I'd like my production to output through the line out of the sound card.  The master bus does not seem to let me switch output at all.  Is this because I am using ASIO driver mode and my Lexicon is the only ASIO compatible device in my system?  Any thoughts how I can send my production to the line out of the sound card?
 
Thanks,
Patrick

First this is the forum for the older versions like 8.5.. 
But your question is universal to all DAWs and interfaces. 
 
It is best to leave things alone and always use ASIO drivers for Sonar. Swapping drivers to MME could result in issues you don't want. Just read half the threads here, they are always about problems with drivers and sound cards. 
 
May I ask why you feel the need to use your on board sound? If it's just to A/B a mix though computer speakers then I would plug them into the headphone jack of the interface if so. 

MME should be fine for those times when you want to send your output somewhere else, presumably to another playback device such as a stereo or PA system. MME is only a problem when you need low latency.
 
Make a note to remind yourself that when the day comes (it will, someday) to replace or upgrade the Lexicon you want an interface with more than one pair of outputs.