All Envy24 based cards have a built in monitor mixer. It can be used to mix all input and output signals together. The settings in the gain. OSS permits using all 10 possible output channels of the monitor mixer even with cards that have less physical outputs. The envy24.rate setting must be set manually to match the actual input sampling rate.
Exact details depend on the soundcard being used.
Use the ossmix command to show a list of the known controls and look at the gain. With some devices it's possible to change the gain controllers to be continuous sliders instead of just enumerated ones. Output routing of output ports can be changed by changing the route. Envy24 based cards have peak meters for the input and output ports of the digital monitor mixer. This increases the system performance significantly.