The SV 112 has a single 8 ohm internal speaker that you plug into speaker output jack "A"

If you want to use a 4 ohm external speaker cabinet, you need to unplug the internal speaker and plug the external speaker into output jack "B"

If you want to use the internal speaker and an 8 ohm external speaker, you need to plug the internal speaker into one of the "B" output jacks and the external speaker into the other "B" output jacks.

There is no muting function associated with the SV 112.  You decide which speakers make sound based on which speakers are plugged in.  And,  you always need to have a speaker properly plugged in any time that the amp is on.

+1 to cgtrox and danielganfield

Or you could always take a look on the product page:

http://www.line6.com/spidervalve/insouts2.html

Regards,

Line6Hugo