For the scenario mentioned in the question, use
ffmpeg -i audio1 -i audio2 -filter_complex \ "[0]atrim=0:10[s1]; [0]atrim=10:20,asetpts=N/SR/TB,volume=0.1[s2]; [0]atrim=20,asetpts=N/SR/TB[s3]; [1]atrim=duration=10,adelay=10000|10000[v2]; [s1][s2][s3]concat=n=3:v=0:a=1[b]; [b][v2]amix[a]" -map [a] mixed.mp3
Usually, you'd want to use the sidechaincompress filter to adaptively reduce the volume of the music stream by analyzing the volume of the foreground audio.