audacity issue on mac

Hello,
I was wondering if anyone has any suggestions regarding volume control of an audio file in audacity. I'm trying to merge two audio files, and lower the volume of one of them, but the volume slider doesn't seem to work with voiceover. I've never done anything like this before, and I'm kind of lost. I couldn't find any info online about this. Any help or suggestions for other apps or software I could use would be greatly appreciated!