You can update order of audio channels and its configuration with "audio_channels" property, but it works for a whole object. In some scenarios, you need to update a configuration of audio channels for a separated frame to process it further with a new configuration.
For this, you can update a frame configuration with MFAVPropsSet method by specifying _bsChannelsRemap parameter. The value can be like the value of "audio_channels" property except for track splitting.
myFrame.MFAVPropsSet(ref myProps, "0,1,2,3");
sets only 4 audio channels in strict order from 1st channel to 4th for your myFrame object. If myFrame originally has more than 4 audio channels the other channels are cut.
myFrame.MFAVPropsSet(ref myProps, "0+1+2,2+3+4");
To create a 2 channels audio from original 4 channels audio the way the 1st new channel contains audio from 1st, 2nd, and 4th original channels, and the 2nd new channel is a sum of 2nd, 3rd and 4th original channels.