PulseAudio

What

Start the daemon as the user you're going to play music/audio as.

pulseaudio -D

Get sinks list/indexes

pactl list short sinks

Set the volume to 100%

pactl set-sink-volume <index> 100%

More help/options:

pactl -h

Change a stream to another sink

pacmd list-sinks # should give you sink indexes
pacmd list-sink-inputs # should give you the stream index
pacmd move-sink-input <stream_index> <sink_index>
pacmd set-default-sink <sink_index>
pacmd set-sink-volume <sink_index> 65536 # equivalent of 100%

You could use the GUI pavucontrol to set this stuff OR the cli stuff above.

Other Links

PulseAudio main page PulseAudio wiki page BAD Ssystem Mode