Set FFmpeg volume mixer with delay functionality
Add the volume filter:
ffmpeg -i s1.mp3 -i s2.mp3 -i s3.mp3 -filter_complex "[1]volume=2,adelay=5s:all=1[a1];[2]volume=0.5,adelay=10s:all=1[a2];[0:a]volume=3[a0];[a0][a1][a2]amix=inputs=3[a]" -map "[a]" output7.mp3