The behavior of channel mapping with single channels depends on the driver (for example, it is different for ASIO and DirectSound on Windows). The best way to be sure you get the desired output is to output one channels with zeros instead of relying on the driver’s channel mapping.
xleftonly = [xmono(:) zeros(length(xmono),1)];
xrightonly = [zeros(length(xmono),1) xmono(:)];
