How to listen live sounds from input from external sound card?

Your problem is likely that the automatically picked source and/or sink for the loopback module is wrong. If you don't specify the source and sink when loading the module, e.g. with:

pactl load-module module-loopback latency_msec=1 source=1 sink=0

then the source and sink are picked automatically. To get the name of your sinks and sources you can use pactl list sources and pactl list sinks. From that output you can use in the above command the number id of the source, or its name, which can be something like alsa_output.pci-0000_00_14.2.analog-stereo.

But the easier way is likely to just simply use

pactl load-module module-loopback latency_msec=1

and then launch pavucontrol, go to the Recording tab, change Show: Applications to Show: All streams. Now you can see from where the loopback module is recording, change it to the source from which your piano sound is coming. Then go to the Playback tab, and do similiarily and change the output of the loopback module to the sound sink where you would like to hear your piano.