Acoustic detection was all the rage before radar was invented; most devices were rather unwieldy, but there were also portable personal systems, like this German one:

https://1.bp.blogspot.com/-M7iDSsoA6_g/V8jmRT8R6AI/AAAAAAAAK4w/5agLXmyGXYImWKRCs5w_nRAyBnkyZHwGgCLcB/s1600/acoustic_locator_13.jpg

where sound tubes were paired with “glasses”, to precisely direct line of sight.

Microphone network would need to have current and precise information about soldiers' relative postitions; I wonder what method would be fast, precise and not easy to spot by itself.