this solves my problem:

pacmd list-sinks

to get list of outputs

then

pacmd set-sink-port sink.primary_output output-wired_headphone

routes audio via jack, and

pacmd set-sink-port sink.primary_output output-line

also routes audio via jack, but the volume is a bit lower.

i guess this is a correct setting.

pacmd set-sink-port sink.primary_output output-speaker

routes back to speaker.

found it by following hints of someone helpful from telegram sailfish chat.