Play audio simultaneously via multiple devices

Basic

pacmd load-module module-combine-sink sink_name=combined

# optional, set as default
pacmd set-default-sink combined

# verify
pacmd list-sinks | grep -e "name:" -e "index:"

To specify the set of devices

# find the names of available sinks
pacmd list-sinks | grep -e "name:" -e "index:"

# create a master virtual sink device
pacmd load-module module-combine-sink sink_name=combined slaves=[name1],[name2]

Latency control

Method 1: add a virtual device

pactl load-module module-null-sink sink_name=delayed
pactl load-module module-loopback latency_msec=150 source=delayed.monitor sink=[fast sink]
pacmd load-module module-combine-sink sink_name=combined slaves=delayed,[slow sink]

Method 2: in the simplest scenario

pactl load-module module-loopback latency_msec=150 source_dont_move=true source=[slow sink].monitor sink=[fast sink]

To combine two HDMI audio devices

check the link below

To remove the virtual combined sink

# when there is only one virtual combined sink
pacmd unload-module module-combine-sink

# when there are multiple, first find out the index
pactl list short modules
# remove the virtual sink by index
pacmd unload-module [module index]

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Based on How to simultaneously play audio via 2 separate HDMI output devices connected to one GPU?, Latency doesn’t work with PulseAudio on linux