Audio Volume decreases after export/upload to Youtube (specifically when listened to on PC)

I have recently been editing interviews that were originally recorded on a DSLR (Mark III 5D, using the built-in audio). When I am editing the videos on my iMac, the audio is loud and clear, but when I export the video (using the Youtube presets), The exported video that I upload to Youtube, has an extremely low volume when played on my clients computer (which is a PC). I have tried messing with the Audio Levels and Gain, but it has cause a ridiculous amount of trial and error to get the volume to an audible level (sometimes even having the Audio level at 6.0dB and gain at 20.0dB). I know in the future the best practice is probably using an external mic, but i want to get these past videos in the right quality for my client. I am fairly new to Premiere Pro so there is probably a step that i am doing incorrectly, but has anyone else ran into this issue/know what is the cause?

Any other audio best practices for Premiere Pro would be greatly appreciated as well!

Thanks in Advance!

Cam