One command to bind them

You can do this in one command using the volume audio filter instead of the old -vol option (as mentioned by your console output):

ffmpeg -i music.mp3 -i videowithaudio.mp4 -filter_complex \
"[0:a]volume=0.390625[a1];[1:a]volume=0.781250[a2]; \
 [a1][a2]amerge,pan=stereo:c0<c0+c2:c1<c1+c3[out]" \
-map 1:v -map "[out]" -c:v copy -c:a libfdk_aac -shortest output.mp4

You may have to adjust the values for volume.

Potential problems

syntax is deprecated

[Parsed_pan_4 @ 0x244c980] This syntax is deprecated. Use '|' to separate the list items.

Don't worry – you can ignore this message.

could not choose their formats

[AVFilterGraph @ 0x26549a0] The following filters could not choose their formats: Parsed_amerge_3
Consider inserting the (a)format filter near their input or output.

All inputs must have the same sample rate and format for amerge. If they are different then you will have to use the aformat audio filter:

ffmpeg -i music.mp3 -i videowithaudio.mp4 -filter_complex \
"[0:a]aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo,volume=0.5[a1]; \
 [1:a]aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo,volume=0.8[a2]; \
 [a1][a2]amerge,pan=stereo:c0<c0+c2:c1<c1+c3[out]" \
-map 1:v -map "[out]" -c:v copy -c:a libfdk_aac -shortest output.mp4