Use the radio buttons in this multichannel block to select between or among sets of stereo inputs and send one set to one of the (2-channel) outputs.
To begin, drag the block into the workspace, right-click it and select Add Algorithm > IC N Stereo/Slew or Stereo. (The result appears the same either way.) The 2 Channel/Slew algorithm lets you rapidly ramp the source level up and down without any clicking noises when switching among outputs. It uses N hardware volume controls in the target/slew RAM to do the job.
The default block (above right) is ready to use as a 2×1 connection, and like the others can be grown by right-clicking to select your desired number of input sets.
See the Multiplexer/Demultiplexer Example utilizing this block.