add Audio to two Video in below command
Use

ffmpeg -y -i intro-17may.mp4 -i video.mp4 -i outro.mp4 -i energy-song.mp3 -i vcs-watermark-video.png -filter_complex "[0:v]drawtext=fontfile=FutuMd.ttf: text='Hello World': x=300: y=450: fontsize=40: fontcolor=white: enable='between(t,2,5)',fade=t=out:st=5:d=1,setpts=PTS-STARTPTS[v0]; \
[1:v][4]overlay=W-w-5:H-h-5,fade=t=in:st=0:d=1,fade=t=out:st=63:d=1,setpts=PTS-STARTPTS[v1]; \
[2:v]fade=t=in:st=0:d=1,setpts=PTS-STARTPTS[v2]; \
[3:a]volume=enable='between(t,34,36)':volume=-8dB,volume=enable='between(t,36,42)':volume=-10dB,volume=enable='between(t,47,51)':volume=-8dB,volume=enable='between(t,51,58)':volume=-10dB,adelay=6000|6000[3a]; \
[v0][0][v1][1:a][v2][2]concat=n=3:v=1:a=1[outv][outa];[outa][3a]amix=inputs=2:duration=shortest[outa]" -vcodec libx264 -c:a aac -ac 2 -map "[outv]" -map "[outa]" c4.mp4


The BG music is delayed by 6000 ms (duration of intro) and then mixed with the audio after concat.