Narration / Voice-over recorded in Movie Maker plays through only one speaker (mono) when listening with headphones on iPhone

Hey all, 

I upload a video to movie maker, I record a narration / voice-over, save the video for youtube. Once I upload to Youtube, the narration plays through the left channel only when listening on my phone with headphones on (the music from the video plays fine through both speakers / channels). 

The resultant mp4 file works great, I can hear the narration through both left and right channels when playing it on my computer (both when listening through on the laptop speakers and with headphones plugged in). But once I upload to Youtube and listen to it on my phone with headphones, narration only comes through the left channel. I use a microphone plugged into the 3.5mm jack of my laptop for recording the narration. I record using the built in narration recorder in Windows Movie Maker. 

How can I record narration where it will play on both channels for those watching the video on a mobile phone using headphones? I don't need stereo narration, just mono being played in both left and right channel would be perfect. Anyone know a solution? I've been researching a solution for hours with no luck. THANK YOU!