Problem when mixing one stereo track with a mono one

Hi, everybody
I am mixing a song in mp3 format, which opens as stereo in Audacity, and a conference which opens as mono.
In the final mix (a stereo wav) there is music in both channels, but the music is louder in the one channel, Why??
Please help!

There is a slider control for each track (whether mono or stereo) which determines the Left-Right balance.
Check that the sliders for both your mono and stereo tracks are in the middle.
If both sliders are in the middle and your one sided problem persists check the mono box is ticked on your mono track …

thank you!