FFmpeg add music to background of video quietly (volume control)

Question

I'm currently able to combine a video file and an audio file. The video has its own audio track.

ffmpeg -i videoAndAudio.mp4 -i backgroundMusic.mp3  -filter_complex "[0:a][1:a]amerge,pan=stereo|c0<c0+c2|c1<c1+c3[out]" -map 1:v -map "[out]" -c:v copy  -shortest output.mp4

The new audio input file drowns out the existing audio from my video.

I'd like to apply volume control to the input audio file, but I don't know how to change the volume of backgroundMusic.mp3.

Thanks