Hi. After changing to surround, did you restart OBS? If you did, could you go into Settings -> Audio and ensure your microphone is still selected in Mix/Aux? If it is, please send a log.
To make a clean log file, first restart OBS, then start your stream/recording for ~30 seconds and stop it again. Make sure you replicate any issues as best you can, which means having any games/apps open and captured, etc. When you're done select Help > Log Files > Upload Current Log File. Copy the URL and paste it in a response below.