Añadiendo esto al build.prop yo es lo que hago
ro.hnd.analog.audio.max=135
ro.hed.analog.audio.max=135
ro.calll.nor.mic=148
ro.calll.hed.mic=152
ro.idle.nor.mic=192
ro.idle.hed.mic=192
ro.calll.hnd.mic=192
ro.idle.hnd.mic=192
ro.def.bt.in.vol=0x500
ro.def.bt.out.vol=0x2000
ro.def.volume.music=10
ro.def.volume.ring=12
ro.def.volume.voice.calll=6
ro.def.volume.system=12
ro.def.volume.alarm=12
ro.def.volume.notification=12
ro.def.volume.fm=11
ro.def.volume.matv=6
ro.def.volume.dtmf=6
ro.def.volume.system.enforced=6
ro.def.volume.bluetooth.sco=6
ro.def.volume.volume.tts=6
ro.nor.sph.vol=64,76,88,100,112,124,136
ro.hed.sph.vol=76,88,100,112,124,136,148
ro.hnd.sph.vol=72,84,96,108,120,132,144
ro.hed.media.vol=32,48,64,80,96,112,128,144,160,176,192,208,224,240,255
ro.hnd.media.vol=32,48,64,80,96,112,128,144,160,176,192,208,224,240,255
ro.hnd.tv.vol=32,48,64,80,96,112,128,144,160,176,192,208,224,240,255
ro.hed.tv.vol=32,48,64,80,96,112,128,144,160,176,192,208,224,240,255
ro.hnd.fm.vol=32,48,64,80,96,112,128,144,160,176,192,208,224,240,255
ro.hed.fm.vol=32,48,64,80,96,112,128,144,160,176,192,208,224,240,255
ro.nor.key.vol=108,132,156,180,204,228,252
ro.hed.key.vol=108,132,156,180,204,228,252
ro.hnd.key.vol=108,132,156,180,204,228,252
ro.hnd.ring.vol=32,48,64,80,96,112,128,144,160,176,192,208,224,240,255
ro.hed.ring.vol=32,48,64,80,96,112,128,144,160,176,192,208,224,240,255