Alesis MULTIMIX 16 Reference Manual page 36

USB2.0 Recording
5
5. Now that the MultiMix is selected as the audio device,
individual channels must be activated for use. Again, return
to the "Devices" menu, and select "VST inputs."
6. Click the activation buttons for channel pairs that you want to
use.
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Some versions of Cubase do
not allow all inputs to be used
simultaneously.