The reason that you’re getting left channel only is because the MA-300 puts one of the xlr channels into the "left" channel, and the other xlr goes into the "right" channel. If you switch the xlr connector to the other jack, you’ll see that all of a sudden the audio is now in the right channel.
The only way around this is to either convert your audio track to mono, or to copy the left channel into the right.
Microphones are mono, so unless you have two mics, one plugged into each jack, you wouldn’t get stereo sound anyway.
DOes that help any? 🙂